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ASTRONOMY 



FOR 



SCHOOLS AND GENERAL READERS. 



BY 



ISAAC SHARPLESS, Sc.D., 

PRESIDENT OF HAVERFORD COLLEGE, 



AND 



GEO. MORRIS PHILIPS, Ph.D., 

PRINCIPAL OF STATE NORMAL SCHOOL, WEST CHESTER, PA, 



FOURTH EDITION— REVISED. 




J. B. L 



PHILADELPHIA: /" V/» 

IPPINCOTT C O M P A N Y. ^J W V A 



! 



X 



OK 



Copyright, 1882, by J. B. Lippincott & Co. 
Copyright, 1892, by J. B. L.ppincott Company. 



PREFACE. 



Astronomy is not studied in the lower and inter- 
mediate schools of the United States as much as its 
importance and interest demand. Its phenomena are 
so striking, so well calculated to awaken thought, and 
so much objects of common notice, that an intelligent 
appreciation of their causes and relations is greatly to 
be desired. 

This book is believed to be written so that any 
person of ordinary education and intelligence can un- 
derstand it. No knowledge of mathematics beyond 
arithmetic is necessary, except that in a few cases trig- 
onometrical solutions of important problems have been 
given in foot-notes for the benefit of those who un- 
derstand such methods. Special effort has been made 
to render ckar the abstruse points in the science, — 
with what success can be judged from the explanations 
of the Transit of Venus, the Precession of the Equi- 
noxes, the Tides, etc. Particular care has been taken 
to distinguish between theories and established facts, 
even when the former seem to be highly probable; 
while mere speculations are altogether excluded. The 
illustrations have been carefully chosen. They are 



PREFACE. 



believed to be better and more numerous than are usu- 
ally found in books of this character, and it is hoped 
that they will render considerable help in making the 
subject clear and interesting. 

The most original feature of the work is the direc- 
tion everywhere given for observations with the naked 
eye and with small telescopes. As illustrations of this 
may be mentioned the methods of observing meteors, 
variable stars, and the phenomena of Jupiter's satellites. 
This plan of setting students at practical work has been 
so successful in chemistry, botany, and other sciences, 
that it seems to be quite time to use it in astronomy. 
It may be that many of the readers of this little book 
will be surprised at the large amount of interesting and 
valuable observation that can be made with the aid of 
a very small glass, and even with the unassisted eye. 



PREFACE TO THE FOURTH EDITION. 



The present edition has been carefully revised. The 
recent discoveries of importance have been included, as 
well as the best of new theories. The authors thank- 
fully acknowledge the receipt of valuable suggestions 
from teachers as to better methods of presenting certain 
portions of the subject, which they have freely used. 
The book is believed to be not only adapted to class 
use, but reliable and thoroughly modern. 



CONTENTS. 



INTKODUCTIOK 

PAGE 

History of Astronomy 9 

General View of the Heavens 18 

Usefulness of Astronomy 26 



PAET I. 

THE SOLAR SYSTEM. 

Chapter I. — General View of the Solar System 28 

H._The Sun ........ 44 

III.— The Inferior Planets 65 

Mercury 66 

Venus 70 

IV.— The Earth 79 

The Tides 118 

y._The Moon 124 

VI.— Eclipses 144 

VII.— The Superior Planets 153 

Mars ,153 

The Minor Planets 159 

Jupiter 163 

Saturn 173 

Uranus ........ 181 

Neptune 183 

VIII.— Comets and Meteors 189 

Comets 189 

Meteors 204 

Relation between Comets and Meteors . .214 
1* 5 



CONTENTS. 



PAET II. 

THE SIDEREAL SYSTEM. 

jPAGE 

Chapter I. — The Constellations . 217 

Description of the Constellations . . . 230 

II.— Double Stars 249 

Variable and New Stars 253 

Clusters and Nebula . . % . . . . 260 

Structure of the Universe 272 



PAET III. 

Properties of Light, and Astronomical Instruments 279 



APPENDICES. 



I. — List of Large Telescopes 
II. — Astronomical Symbols 
III. — Lengths of Days, Months, and Years 
IV. — Statistics of Planets, Sun, and Moon 

V. — Periodic Comets 

VI. — List of Noted Double Stars . 
VII. — List of Noted Clusters and Nebulae 



305 
306 
307 
308 
309 
310 
311 



SUGGESTIONS TO TEAOHEES. 



Aids. — A celestial globe, twelve inches, or there- 
abouts, in diameter, is most useful in illustrating 
and explaining many astronomical phenomena, and in 
finding the constellations and principal stars. Be 
sure that the globe has a horizontal ring about the 
middle. A Planisphere is a tolerable substitute for a 
globe, and much cheaper. A Star Lantern is also very 
convenient. A good star-map is important. A tele 
scope of any size, or even a good spy-glass or pair of 
opera-glasses, will add much interest to the study. 

Methods of Instruction. — Each teacher has his own 
method of conducting recitations, but the authors' ex- 
perience leads them to prefer the topical method, and 
whenever possible they would have the student learn 
the topics in their order, so as to get a complete and 
connected knowledge of the subject. The headings of 
the paragraphs and the arrangement of the topics will 
facilitate this. Eeviews here, as elsewhere, will be 
found to be very valuable. Distances, dimensions, etc., 
as given in round numbers, should be learned and 
made perfectly familiar by frequent repetition. Par- 
ticular attention ought to be paid to the questions and 



8 SUGGESTIONS TO TEACHERS. 

suggested problems in the foot-notes. This will test 
the pupil's knowledge and make it more thorough. 
A teacher should never be content until his class un- 
derstands each point thoroughly ; and it must not be 
forgotten that no one can explain clearly to a class 
what he does not clearly understand himself. It is 
hoped that every teacher who essays to teach this 
subject will acquaint himself thoroughly with it by 
making use of standard works upon astronomy ; and 
he may rest assured that no knowledge that he can 
acquire will be more interesting, or more valuable 
everywhere and anywhere, than this. 

Practical Work. — Above all things, the teacher 
must not neglect the practical work. Let him take 
his class out under the clear sky and point out the 
constellations, principal stars, and planets. Let him 
make himself, and lead his students to make, the ob- 
servations described in the following pages. He will 
be surprised at the interest awakened, and at the valu- 
able results. A common household almanac will be 
of great aid here. There is much more in an almanac 
than most people see. 



INTRODUCTION. 



History of Astronomy. 

1. Early History. — Astronomy, the science of the 
heavenly bodies, is probably the oldest of all the sci- 
ences. So old is it that there is no trustworthy account 
of its origin ; indeed, almost every famous nation of 
antiquity claimed the honor of originating it. Nor is 
it hard to see why this science should have been culti- 
vated so early. The first men had no books to occupy 
their time, hence they observed nature. The most 
striking occurrence was the succession of day and 
night, the one lighted up by the brilliant sun, the other 
dark, or feebly illuminated by the wonderful stars and 
the curiously changing moon. These changes were a 
very natural division of time, the only ones they had. 
As men knew not the true God, they naturally turned 
to the heavens for objects of worship, and this led to 
careful study and observation of the heavenly bodies 
by priests and other ministers of religion. Besides, 
the occupations and modes of living of our earliest an- 
cestors were most favorable to the study of astronomy. 
As hunters, shepherds, and farmers, their lives were 
spent in the open air, by night as well as by day. In 
travelling over the thinly-peopled earth, and the sea as 

9 



10 INTRODUCTION. 



well, the stars were their guides. It is not surprising, 
then, that these men, with no instruments, no books, 
no schools, knew much about astronomy. They seem, 
in fact, to have known more about the appearance and 
phenomena of the heavens than we generally do. 

2. Astronomy of the Chaldeans. — According to the 
Greek historians, the Chaldeans were the first astrono- 
mers. These people lived along the Euphrates River 
in Asia, in and about the city of Babylon. They kept 
careful records of the movements and phenomena of 
the heavenly bodies. By these records they discovered 
that the eclipses of the sun and moon are almost ex- 
actly repeated every eighteen years, and thus success- 
fully predicted eclipses. But of the real causes of 
eclipses, or of the nature, distance, or real motions of 
the heavenly bodies, these ancient astronomers knew 
nothing. 

3. Astronomy among other Ancient Nations. — The Egyp- 
tians, like the Chaldeans, studied astronomy in very 
ancient times. Some writers contend that their famous 
pyramids are so built as to show great astronomical 
knowledge, but very little is certainly known about 
this matter, or about their advancement in astronomy. 
It seems to be proved that the Chinese had a knowl- 
edge of astronomy very early, — more than four thou- 
sand years ago, according to their own claim ; but the 
evidence of this extreme age of the science among them 
is doubtful. Their records relate that about that date 
Ho and Hi were the two royal astronomers, whose duty 
it was to predict all eclipses, but that, giving themselves 
up to the pursuit of pleasure, they neglected their du- 
ties, and an eclipse of the sun occurred without being 
predicted. The whole nation was thus exposed to the 



IXTRODVCTIOX. \\ 



anger of their gods, because of the omission of the 
religious ceremonies always performed upon such oc- 
casions. The unfortunate astronomers were imme- 
diately put to death. It is certain, however, that the 
Chinese made reliable astronomical observations, some 
of which are of use to us, at least two thousand five 
hundred years ago. 

The Hindoos also claim to have been the first to 
study astronomy. They have proved their claim to an 
extensive knowledge of the subject, but whether they 
borrowed this knowledge from the neighboring nations, 
or gained it by observation, is uncertain. 

4. Greek Astronomy. — Astronomy was a favorite sci- 
ence with the ancient Greeks. But, as was the case 
with a great part of their science, their astronomy was 
imagined rather than observed. Some of their astron- 
omers advanced surprisingly correct theories of the 
heavenly bodies, but seem to have made little effort 
to prove them. Certain of their earliest philosophers 
taught that the earth is a sphere, a belief not original 
with Columbus, as some people think, but one taught 
in Greece two thousand years before Columbus was 
born. Later some of the Greeks taught that the sun 
is the centre of the system of planets to which the earth 
belongs, and that all revolve about the sun ; others 
taught that day and night are caused by the revolution 
of the earth upon its axis. These great truths are the 
foundation of modern astronomy, but the Greek phi- 
losophers brought forth so little evidence in support of 
these guesses, and mingled so many absurdities with 
them, that they were not generally believed, and were 
soon forgotten. Notwithstanding their general habit 
of neglecting experiments for theories, the Greeks 



12 INTRODUCTION. 



achieved some substantial results. They made obser- 
vations which were of use to succeeding astronomers, 
they greatly improved the reckoning of time, and de- 
termined the length of the year to be three hundred 
and sixty-five and one-fourth days, which is wonder- 
fully near its exact length. 1 

5. The Alexandrians. — For a few hundred years be- 
fore and after the Christian era, 2 the city of Alexandria 
in Egypt was famous for its learning. Its astronomers 
were the most skilful that had yet lived. They at- 
tempted to find the relative distances of the sun and 
moon from the earth. The method employed was a 
correct and very ingenious one, but from the imperfec- 
tions of their observations their results were far from 
the truth. They determined the width of the torrid 
zone with great exactness, and found the circumference 
of the earth with surprising accuracy, using the method 

1 The ancients found the length of the year by means of a gnomon 
(no'mon). This was a pillar set up to cast a shadow, which was 
measured at noon every day. When the noonday sun was lowest 
down in the sky the shadow of the gnomon was longest, as a little re- 
flection will show. This time of year is called the winter solstice, 
and marks the time when the sun is farthest south of the equator and 
is shining directly down upon the tropic of Capricorn ; according to 
our reckoning, this is about the 21st of December. After that date 
the noonday shadow grows shorter, because the sun gets farther north 
every day. Now, if the day upon which the gnomon's shadow is 
shortest is found, and the days are carefully counted until the short- 
est shadow comes again, the length of the year is found. It is inter- 
esting to know that the obelisks of Egypt, one of which has lately 
been brought to New York and set up in the Park there, are thought 
to have been used as gnomons. 

If the gnomon were south of the equator, would it make any change 
in this explanation ? Could the time of noon be found by measuring 
the length of the shadow ? 

2 What is meant by this ? 



INTRODUCTION. 13 



which is still used as the very best one known. It 
will be described farther on in the book. Euclid, 1 who 
gave us the geometry which in substance is still uni- 
versally used, lived in Alexandria during this period, 
and contributed to the advancement of astronomy. 

6. Hij)par chics. 2 — This was the greatest of the ancient 
astronomers, and well deserves his title, " Father of 
Astronomy." He lived upon the island of Rhodes, in 
the Mediterranean Sea, about 150 B.C. 3 Hipparchus 
determined the length of the year to within about four 
minutes of its true length. He discovered that the 
distance from the sun to the earth varies throughout 
the year, and he made several most important discov- 
eries in the movements of the heavenly bodies. He 
made the first catalogue of the stars, fixing the position 
of over a thousand of them. This catalogue is one of 
the most valuable possessions of modern astronomy. 
Hipparchus invented the science of trigonometry, and 
first used latitude and longitude to determine the posi- 
tions of places on the earth. 

7. Ptolemy* and his System. — This most famous astron- 
omer of antiquity lived at Alexandria about 130 a.d. 3 
He made few observations himself, but collected the 
results of other men's work and wrote them down, to- 
gether with some important investigations of his own, 
and it is to him that we owe almost all our knowledge of 
ancient astronomy. His great work upon astronomy, 
the " Almagest," still exists, and for fourteen hundred 
years it was the highest and the only authority upon 

1 Euclid (yoo'klid) nourished about 300 B.C. 

2 Pronounced Hip-ar / kus. 

8 What is meant by this? How long ago were these times ? 
* Pronounced Tol'e-my. 

2 



14 INTRODUCTION. 



the subject. The foundations of the Ptolemaic system 
are that the earth is a sphere, that it is the centre of the 
universe, and that it is stationary, while all the heavenly 
bodies revolve about it every twenty-four hours. That the 
earth is a sphere Ptolemy proved by the fact that at 
places west of the observer the sun rose and set 
later, and at places east, earlier ; l and also because as 
one goes north the pole-star rises higher in the sky, 
while it sinks lower as he goes south. That the earth 
stands still while the sun and stars revolve about it, 
Ptolemy argued was simply common sense. And he 
took some pains to show the absurdity of the belief 
that these phenomena are caused by the turning of the 
earth upon its axis. Ptolemy's theory explains the 
apparent motions of the sun, moon, and stars pretty 
well, but the apparent motions of the planets 2 are so 
peculiar, as will be explained when these are treated 
of, that he was forced to conclude that these bodies do 
not move in circles about the earth, but in very com- 
plicated circular paths, composed of series of loops. 
This is the theory of the universe which was accepted 
everywhere without question until the sixteenth cen- 
tury. 3 

1 A little thought, aided perhaps by a diagram, will make this rea- 
soning clear. At St. Louis the sun rises and sets an hour later than 
at Philadelphia ; hence St. Louis time is an hour behind Philadelphia 
time. How would this affect travellers ? How does it affect railroad- 
trains ? 

2 A few of what are commonly called stars are planets, and are 
comparatively near to us. They resemble the earth in many respects. 
The others are properly called stars, and are suns, situated at immense 
distances from us. The word planet is derived from a Greek word, 
meaning a wanderer, because these bodies wander among the stars, 

3 The sixteenth century began at the beginning of the year 1501 



INTRODUCTION. 15 



8. Copernicus 1 and his System. — It has already been 
mentioned that some of the old Greek astronomers 
held and taught the true theory of the heavenly bodies, 
but, substantiated by no proofs and borne down by 
the great authority of Ptolemy, their teachings had 
long since been forgotten. And it was not until about 
1500 a.d. that Copernicus, a Prussian mathematician 
and astronomer, revived and firmly established the 
essential truths of astronomy. He showed that the 
earth and planets revolve about the surf as a centre, 
and that the daily risings and settings of the heav- 
enly bodies are caused by the turning of the earth 
upon its axis. Although his theories were not strictly 
original with him, and although he left them very 
incomplete, yet Copernicus has been honored greatly 
and justly for bringing forward and clearly stating 
the true principles of astronomy, at the same time 
showing good reasons for his belief; as well as for 
his courage in thus breaking away from the ignorance 
and superstition of his age. His work upon the subject 
was not published until just at the close of his life, and 
the first printed copy of it was put into his hands only 
a few hours before his death. In his honor our theory 
of astronomy is still called the Copernican System. 

9. Kepler. 2 — This great mathematical astronomer 
followed Copernicus. His whole life was spent in 
laborious calculations. His name is most frequently 
mentioned in connection with three great laws, which 
explain the paths, motions, and distances of the planets. 

and ended at the close of the year 1600. "What century is this ? 
When did it begin, and when will it close ? 

1 Copernicus (ko-per'ni-kus), 1473-1543. 

2 Kepler, a German, 1571-1630. 



16 INTRODUCTION. 



These three laws (see page 36), which would scarcely 
fill a half of one of these pages, cost him seventeen 
years of hard work. When the third one was estab- 
lished, he said of the book containing it, " It may well 
wait a century for a reader, as God has waited six 
thousand years for an observer." 

10. Galileo. 1 — This famous Italian first used the tele- 
scope in astronomy. The first telescope was made 
in Holland in 1608; a vague report of the invention 
reached Galileo the next year, and from this hint, after 
one night's reflection, he was able to construct one 
which magnified objects three times, and he finally 
made one which magnified thirty-two times. He dis- 
covered the moons of Jupiter, the spots upon the sun, 
and many other wonderful things. His brilliant dis- 
coveries convinced the world of the truth of the Co- 
pernican theory, but brought on him the condemnation 
of the Church for teaching heresies, and the closing 
years of his life were saddened by its persecutions. 
Natural philosophy is as greatly indebted to this re- 
markable man as astronomy. 

11. Newton. 2 — Within a year of the day on which 
Galileo died, Sir Isaac Newton was born in England. 
While Newton did not discover the law of gravitation, as 
is sometimes stated, yet he first proved that the force 
which brings the apple to the earth binds the planets 
and sun into one system. 3 This establishment of the 

1 Galileo (Gal-i-lee'o), 1564-1642. 

2 New'ton, 1642-1727. 

3 The well-known story that while driven into the country by the 
Plague in London, Newton noticed an apple falling from a tree, and 
that this suggested the idea that the motions of the planets might be 
controlled by the same force, is worth remembering. This discovery 



INTR OD UCTIOK 1 7 



fact that gravity is the force which controls the motions 
of the heavenly bodies was of the greatest importance : 
a large part of the science of astronomy depends upon 
it. 1 Newton, like Kepler, was a mathematical astron- 
omer, not an observer. He discovered and proved 
many other important facts in astronomy, besides 
making many and valuable discoveries in natural phi- 
losophy and other sciences. He also occupied impor- 
tant positions under the English government. Sir 
Isaac Newton was probably the greatest scientist that 
the world has yet seen. His great work is called the 
" Principia." La Place, 2 the only man who could have 
disputed Newton's pre-eminence as a mathematical as- 
tronomer, pronounced this work the greatest produc- 
tion of the human intellect. 

12. Modern Astronomy. — Since Newton a host of emi- 
nent astronomers and mathematicians have given their 
lives to the advancement of our science. Every gen- 
eration and every civilized country has furnished its 
share. As it was the earliest begun, so it is the far- 
thest advanced of the sciences. Its strides seem to be 
growing longer rather than shorter. Our own genera- 
tion and our living astronomers are inferior to none of 
their predecessors in ability or in the value of their dis- 
coveries. And there is every reason to expect these 
discoveries to go on with increased rapidity. In the 

was made by Newton while he was absent from London on account 
of the Plague, but the rest of the story is not supported by sufficient 
evidence ; it is not at all improbable, however. 

1 It may not be amiss to remark that, while the laws and effects of 
gravity are well known, the cause of this force has never been dis- 
covered. 

2 La Place (La-plass'), a great French mathematician and astrono- 
mer, 1749-1827. 

2* 



18 INTRODUCTION. 



astronomical work of the last generation our own 
country has done its full share. Our astronomers and 
observatories have no superiors. Our contributions to 
the world's store of knowledge have been greater in 
this direction than in any other. The history of the 
important discoveries in astronomy made since New- 
ton's day would fill a much larger book than this. We 
can only give these discoveries in their proper places 
in a general account of the subject. 



General View of the Heavens. 

13. Introductory. — If a person will carefully watch the 
heavens, he will see much that will tend to excite his 
curiosity. What are all the glittering lights? How 
far are they away ? Why do they seem to move around 
him in a circle every day? Why do some of them 
change their position among the others ? Many such 
questions as these will come up, and the best method 
of arriving at a correct answer is first to observe care- 
fully all that can be seen. The ancients did this much 
more faithfully than we do, and the various generations 
of men have accumulated a great number of facts and 
laws of which we can now have the benefit. % It is the 
object of a book on astronomy to explain these points 
so that an observer can better comprehend the causes 
of what he sees. But careful watching must accom- 
pany the study if the phenomena are to be fully under- 
stood. 

14. The Heavens by Day. — But what can the unaided 
eye see ? In the daytime there is usually only the sun, 
and this presents the same general appearance every 



INTRODUCTION. 19 



day. We will find that continual changes are taking 
place on his surface, but these changes are not visible 
to the eye. His position in the heavens is, however, 
perceptibly changing. Every one is familiar with the 
motion which occurs each day, — his rising in the east, 
reaching the highest point at noon, and setting in the 
west. A careful observer will notice, besides this, a 
change of place at different seasons of the year. He is 
in the south every day at noon, but in the summer he 
is higher up in the sky than in the winter. It will be 
noticed, too, that he does not rise and set in the same 
place through the year. If the point of setting be 
noted every evening, beginning with the first of the 
year, it will be found to be moving towards the north 
as the winter progres&es. This will go on till the mid- 
dle of summer, when the place of setting will be far to 
the north of the west. Then it will slowly change back 
again towards the south through the fall and early 
winter. So with the time of rising and setting : it will 
be noticed that the farther to the north the sun rises, 
the earlier in the day it will rise and the later it will 
set. 1 

15. Horizon and Zenith. — The circle where the earth 
and sky appear to meet is called the horizon. On the 
ocean it is a perfect circle, but on land it is broken up 
with the irregularities of the surface. When the sun 
rises it passes above this circle, and when it sets it sinks 

1 Let the student carefully note, by reference to a tree or some dis- 
tant object, the point in the horizon where the sun rises or sets, at 
intervals of a week or two, and this change of place will be readily 
manifest. He must be careful to occupy the same point of observa- 
tion at the different times. Let him also with a watch observe the 
exact time, and thus notice the gradual change. 



20 1NTR OD UCTIOK 



below it. The point in the sky directly over the head 
of the observer is called the zenith. 1 

16. The Heavens by Night — In the night there is 
much more to attract attention in the sky. The 
moon seems to follow nearly in the path of the sun. 
If carefully observed, she will be seen to change her 
place among the stars, being each night a little farther 
to the east than the preceding. The changes in her 
appearance from crescent-shaped to full, and from full 
to crescent-shaped, are also striking. There will be 
certain nights each month when she cannot be seen ; 
after this a glimpse of her can be obtained in the west 
just after sunset; she will then be crescent-shaped, and 
her horns will point directly away from the sun. She 
will then grow in size for about two weeks, all the 
time appearing farther and farther away from the sun 
at sunset, till when quite full she will rise in the east 
just as the sun is setting in the west. Then she will 
go through the changes in a reverse order for two 
weeks more. 

It will also be noticed that certain of the brighter 
stars appear, like the moon, to change their places 
among the others. The ancients called these planets, 
or " wandering stars." Those that can readily be seeri 
by the naked eye are* Venus, Mars, Jupiter, and Saturn. 

But the great majority of the stars preserve exactly 
their relative positions. They appear night after night 
looking precisely the same. A given star will always 

1 Towards what point will a plumb-line, extended upwards, point? 
Does the zenith change with a change of position on the earth ? Is 
the sun ever seen in the zenith in the northern hemisphere ? On 
which side of the zenith is the sun at noon ? In what time of year 
does the sun pass nearest the zenith ? 



INTRODUCTION. 21 



rise in the same point in the horizon, though not at 
the same time ; it will always follow in the same path 
throughout the year, and set in the same place in the 
west. 

But it will be noticed that the paths which different 
stars describe are very different. If we look in a north- 
erly direction towards a point nearly half-way from the 
horizon to the zenith, we shall see a star of medium 
brightness which does not change place at all ; it is the 
pole-star, or Polaris. Around this star all the northern 
heavens seem to revolve in circles. If these northern 
or circumipolar stars be watched, such as are between 
the pole-star and the horizon will move towards the 
east ; such as are on the east of the pole will ascend ; 
such as are above will move westward ; and such as are 
to the west of the pole will descend. Those stars situ- 
ated a little farther from the pole than the northern 
horizon is will just dip below it and remain set but a 
short time. Those that rise in the east will be visible 
just twelve hours and set in the west; they will not, 
however, pass through the zenith, but south of it, 
always remaining the same distance from the pole- 
star. Still farther to the south the stars will be but a 
short time above the horizon, passing over from south- 
east to southwest. 1 

1 In order to obtain a correct idea of this diurnal motion, the stu- 
dent should watch stars in different parts of the heavens at intervals 
of a few hours, so as to notice the paths they are describing. It is 
also advisable to set a globe so that the axis about which it revolves 
will point nearly to the pole-star. The horizontal ring encircling the 
globe will then represent the horizon. By turning the globe on its 
axis, it will be seen that the part around the pole will not pass below 
the horizon, and the various circles of latitude will represent the 
paths of stars in different parts of the sky. Some portions around 



22 INTRODUCTION. 



17. Diurnal Motion. — This general motion of the sun, 
moon, planets, and stars, which carries them apparently 
around the earth every twenty-four hours, is called the 
diurnal motion. The heavens appear to us to be the con- 
cave surface of a sphere, called the celestial sjihere. The 
celestial sphere and all the heavenly bodies revolve 
about the earth every day, while the sun, moon, and 
planets have a separate motion of their own, which 
causes them to change their places among the stars. 

18. Cause of Diurnal Motion. — A quiet motion often 
gives the impression of rest. A sailing vessel will 
glide along through still water so quietly that a person 
on board can easily conceive that he is at rest and sur- 
rounding objects are in motion in the opposite direc- 
tion. Now the earth is turning on its axis from west 
to east with a perfectly noiseless and smooth motion. 
The effect produced on us is that all the heavenly 
bodies are passing over from east to west The appar- 
ent diurnal motion of the heavens is therefore due to 
a real motion of the earth. Instead of the sun, moon, 
and stars rising above the horizon, the eastern horizon 
is really falling away from them. Instead of their set- 
ting, the western horizon is rising to obscure them. 
The reason that they appear to climb the sky is because 
the portion of the earth on which we are is turning 
more directly under them ; and the reason that they 
sink is because we are revolving away from them. 
All the effects of diurnal motion above described are 
readily explained by the rotation of the earth on its 
axis, this axis pointing nearly towards the pole-star. 



the south pole will not pass above the horizon. There are some very 
brilliant southern stars that we never see in this latitude. 



INTRODUCTION. 23 



In some explanations it is easier to consider that the 
sun moves about the earth, as it seems to do. When 
we speak in this way, it must be remembered that we 
refer to the apparent and not the real motion. 1 

19. Celestial Measures. — The heavenly bodies being 
apparently on the inner surface of a sphere, the line 
joining their positions on this sphere is an arc of a 
circle. Hence we do not measure distances in the 
heavens by miles or other linear units, but by circular 
measure. Every circle is divided into three hundred 
and sixty degrees (°), each degree into sixty minutes ('), 
and each minute into sixty seconds ( /; ). The distance 
from the zenith to the horizon is a quarter of a circle, 
or ninety degrees. It is well 
for the student to have a correct 
idea of the size of small meas- 
ures in the heavens. The fol- 
lowing will aid in obtaining it. 
There are two stars which con- 
tinually point to the pole-star, 



Pointers 

MSA ,,> 

AtAJtffL 
i 
I 



They are two of the seven Fl0,1, 

which form what is often called the Dipper, revolving 
continually about the pole-star and just touching the 
northern horizon. These two " pointers" are just 

1 In the summer of 1881 there was a bright comet, the tail of which 
pointed nearly to Polaris. It partook of the diurnal motion of the 
heavens, and being near the pole-star was seen all night. When 
in the northwest in the evening, its tail pointed upwards and to the 
right. When it got around to the northeast in the morning, the tail 
pointed upwards and to the left. Many people who saw it in both 
of these positions, not understanding about the diurnal motion, 
thought there were two comets. Let the student think of this matter 
till he sees how it was that the comet thus changed the direction of 
its tail with reference to the horizon. 



24 INTRODUCTION. 



about five degrees apart. The diameter of the sun 
and that of the full moon are each about half a degree 
or thirty minutes long. If two stars are nearer to- 
gether than three or four minutes, they will appear as 
one to the eye. 1 

20. The Heavens at the Equator and at the Poles. — As 
the observer changes his position on the earth, the ap- 
pearance of the heavens will also change. If he move 
eastward or westward, his horizon will move the same 
way, and the time of rising and setting of the stars 
will vary. If the movement be eastward, the same 
stars will pursue the same course through the sky, but 
they will rise earlier and set earlier ; if westward, the 
reverse will be the case. If, however, the observer 
move towards the north or south, the whole aspect of 
the heavens will change. 

The reason that the pole-star does not seem to move 
is because the axis about which the earth revolves 
points almost directly towards it. 2 There is no change 
of the horizon with reference to it. To an observer at 
the equator the pole-star would be at the horizon, be- 



1 The following additional measurements will assist in estimating 
distances. The stars may be found on a map or globe, or some one 
knowing them may point them out in the heavens. The extreme 
stars of the three in the belt of Orion are about three degrees apart ; 
Castor and Pollux about four degrees. Near Vega is a faint star, 
which by a good eye can be seen to be made up of two stars. They 
are three and a half minutes apart. The height of the pole-star above 
the horizon is about equal to the latitude of the place. In the Middle 
States this is nearly forty degrees. 

2 The axis of the earth does not point directly towards the pole-star, 
but about one and a half degrees from it. The pole-star, therefore, 
describes a small circle about the pole of the heavens, though, roughly 
speaking, it may be said to correspond with it. 



INTRODUCTION. 25 



cause the axis of the earth is pointing in that direction. 
If a globe be set with its axis horizontal, it will show 
the motion of the heavenly bodies to a person at the 
equator. The stars that rise in the east will pass di- 
rectly overhead and set in the west ; every star will be 
just twelve hours above the horizon ; those around the 
poles will describe small circles, those farther away 
larger; there will be no stars that never rise, and none 
that never set. 

As the person moves towards the north pole, the pole- 
star will rise above the horizon, its height being equal 
to the latitude of the person; 1 that is, if the observer 
is at latitude 40°, as at Philadelphia, the pole-star will 
be forty degrees above the horizon. When the ob- 
server reaches the pole, the pole-star will be in his 
zenith ; the heavens seem to move as in the case of a 
globe with its axis vertical; only one-half the stars 
will be ever visible: those in the horizon will con- 



1 The elevation of the pole-star may be shown to be equal to the 
latitude by the aid of Fig. 2. 

Let BAC be a meridian of the earth, P the north pole, and E the 
point where the meridian cuts 
the equator. The axis of the 
earth, OP, will cut the celestial 
sphere at a point, P / , very near 
the pole-star. Let A be the posi- 
tion of the observer on the earth. 
Then the arc EA, or the angle 
EOA, is the latitude of the place 
of the observer, and BOP is the 
elevation of the pole of the heavens above the horizon. We wish to 
prove that EOA = BOP. 

BOA is a right angle, as is also POE, because the equator is ninety 
degrees from the pole. Hence BOA = POE. Taking from these 
equals the angle POA, we have BOP = AOE, which is what we 
wished to prove. 




26 - INTRODUCTION. 

tinually skirt around the horizon ; none will ever rise, 
and none set. 

The same changes would be noticed if the observer 
moved southward from the equator, except that there 
is no star to mark the position of the south pole. 



Usefulness of Astronomy. 

21. Astronomy, besides being a very grand and 
interesting science, has great practical usefulness. 

Every day there is telegraphed over the country from 
the Washington and other observatories the accurate 
time of noon ; this is determined by astronomical ob- 
servations, without which it would be almost impos- 
sible to keep our clocks and watches correct. 

Every captain of a vessel when he starts out on a 
long voyage takes with him a chronometer 1 which has 
been previously tested at an observatory, and a nautical 
almanac? in which the positions of the sun, moon, and 
principal stars are given with great accuracy. With 
these and some simple observations he is able to tell 
his position on the ocean and thus to direct his move- 
ments. 

The basis of our calendar is astronomical. The 
lengths of the year, month, and day are governed by 
phenomena of the heavenly bodies, and are determined 
by observations of them. Our common almanacs are 
calculated from the nautical almanacs, which are issued 
from the national observatories. 

1 A clock swinging in rings, so that the motion of the vessel will 
not affect it. 

2 This will be further explained on page 169. 



INTRODUCTION. 27 



All the maps of the surface of the earth are depend- 
ent for their accuracy on astronomical observations; 
the methods of finding latitude and longitude are 
largely astronomical. 

Astronomy is also a help to geology, to meteorology, 
and to other sciences. 

Hence we see that it is one of the very practical 
sciences ; and it will probably be found that some of 
its researches, which do not now seem to be of any 
use to man, will in the future be in some way closely 
related to his welfare. 

It will be of use to students also, if they study it 
rightly, to teach habits of observation, to strengthen 
their powers of thought, and to give correct ideas of the 
method by which the Creator of the universe works. 



PART L 

THE SOLAR SYSTEM. 



CHAPTER L 

GENERAL VIEW OF THE SOLAR SYSTEM. 

22. Parts of the Solar System. — The group of bodies 
to which the earth belongs is called the solar system. 
It consists of the sun, the planets, their satellites or 
moons, the comets, and meteoroids. 1 The earth is 
one of the planets, and the moon one of the satellites. 
These bodies are closely connected with one another, 
and, comparatively speaking, are close together. The 
sun is very much the largest and most important 
member of the system : hence the name solar 2 system. 
The stars are all situated at immense distances from 
us, and, aside from their light, exert little or no in- 
fluence upon us. 

23. Arrangement of the Solar System. — The sun is the 
centre of the solar system, and about it all of the other 



1 "Shooting stars" are meteoroids which have come into our at- 
mosphere. 

2 Solar, from Latin sol, the sun. 

3* 29 



30 



ASTRONOMY. 



members revolve. The time that it takes one of these 
bodies to revolve about the sun is called its year. If 
an observer could be at the sun and watch the other 
members of the solar system, they would revolve about 
him in apparent circles, just as we see the moon re- 
volving about the earth. But from one of the planets 
these motions do not seem so simple, and it was a long 
time before men found out that the earth and the rest 
of these bodies revolve about the sun. 




Fig. 3. — The Orbits of Maes, the Earth, and Venus. One inch = 100,000,000 miles. 
The arrows show the direction in which the planets move, as seen from the north side 
of their orhits. 



The path in which a body moves about the sun is 
called its orbit. Fig. 3 shows the orbits of the earth 
and the planets next to it on either side, Mars and 



GENERAL VIEW OF THE SOLAR SYSTEM. 31 

Venus. Those planets whose orbits are inside of the 
earth's orbit, as Venus, are called inferior planets, be- 
cause they are nearer to the sun. Those outside are 
called superior planets. 

24. Positions and Apparent Motions. — When a heav- 
enly body is on the side of the earth opposite to the sun, 
it is said to be in opposition; thus, if Mars is at M, with 
the earth at E, Mars is in opposition. "When a heav- 
enly body and the sun are on the same side of the 
earth, the body is in conjunction ; thus, if Mars is #,t M', 1 
with the earth at E, Mars is in conjunction. It is evi- 
dent from the figure that an inferior planet has two 
conjunctions. With the earth at E, Venus at V is in 
inferior conjunction, but at V 7 is in superior conjunction. 
If in going between the earth and the sun Venus 
should happen to pass directly across the face of the 
sun, it would be called a transit. 2 This rarely happens; 
the inferior planets usually cross a little below or above 
the sun. A superior planet may be seen at any height 
in the heavens; it may be in opposition to the sun, 
when it would rise about the time the sun sets, and 
would shine all night. An inferior planet can never 
be in opposition to the sun, but in revolving about the 
sun seems to us to pass back and forth, from one side 
of the sun to the other, as Fig. 3 shows is the case with 
Venus. An inferior planet, then, is never far from the 
sun ; it is only seen a little while after sunset or before 
sunrise. When Venus is at V" or V /;/ , with the earth 
at E, it seems to be farthest from the sun ; it is then 
said to be at its greatest elongation. The planets all 

1 M' is read M prime ; V', Y prime ; Y 7/ , Y second ; V /// f Y 
third, etc. 

2 Can a superior planet ever transit ? 



32 ASTRONOMY. 



move about the sun in the same direction, from west 
to east. To an observer north of them (as anywhere 
in the United States north of Florida) they would seem 
to move from the right over to the left, or in a direc- 
tion opposite to the motion of the hands of a clock. 1 
Although the planets always move around the sun in 
the same direction, our position upon the earth makes 
them seem to move differently sometimes. "With the 
earth at E, Yenus seems, while moving from V" to 
V //; , to move in the proper direction, from right to 
left, but while moving from Y fn to V", across between 
us and the sun, it seems to move in the opposite direc- 
tion. 2 This is called its retrograde (backward) motion. 
A superior planet retrogrades when the earth passes 
between it and the sun. The earth leaves the planet 
behind, and it seems to move backward, just as trees 
seem to move backward when we pass them in the 
cars. If we imagine ourselves at E, watching Venus 
pass us, or Mars as we pass him, it will be clear. 

25. Shapes of the Orbits. — The orbits of the planets 
are not circles, but ellipses. An ellipse is an oblong 
curve, so made that the sum of the distances from any 

1 This motion must not be confounded with the apparent diurnal 
motion of the heavenly bodies. AW of the planets and stars seem to 
move every night from east to west, which, as has been explained, is 
caused by the revolution of the earth upon its axis. But the motion 
here referred to is one which the planets have in the opposite direction 
among the stars, just as the moon moves to the east among the stars, 
although it rises and sets with them. The planets move more slowly 
than the moon, but if one of them be watched from night to night, 
its motion eastward among the stars may be seen. It is very im- 
portant to have this matter perfectly clear. 

2 For the sake of simplicity the earth is here supposed to be station- 
ary ; the earth's motion really shortens very much the time of retrogra- 
dation. 



GENERAL VIEW OF THE SOLAR SYSTEM. 33 



point of it to two fixed 'points is always the same. Fio\ 4 
represents an ellipse. The sum of ES and EF is just 
equal to the sum of E'S and E'F. 1 If the ends of a 
string be fastened at two points (S and F) upon a table, 
so as to lie loosely between them, and a pencil held 
against the string so as to stretch it (as at E) be moved 





Fig. 4.— Ellipse. 



Fig. 5.— Parabola. 



along, it will mark an ellipse. S and F are called the 
foci (fo'si). In the orbits of the planets the sun is al- 
ways at one focus (fo'kus). If the foci are nearer to 
the centre C, the ellipse is nearer circular. The eccen- 
tricity of an ellipse is the distance CS divided by CP ; 
it is usually expressed in a decimal fraction : the eccen- 
tricity in Fig. 4 is .8, or f . 2 The eccentricity of an 
ellipse shows whether it is nearly circular or more ob- 
long. The orbits of the planets have very little eccen- 
tricity, as the table in Art. 27 shows. It must be re- 
membered, then, that the elliptical shape of a planetary 



1 Measure the lines in the figure, and see if SE and EF taken to- 
gether are equal to SE 7 and E'F. Try the sum of the distances to 
any other point on the curve. 

2 Measure CS and CP, and see if CS is .8 (or f ) of CP. 



34 ASTRONOMY. 



orbit, as shown in Fig. 4, is greatly exaggerated. An 
exact figure of a planet's orbit could not be distin- 
guished by the eye from a circle. Fig. 3 shows the 
real shapes of the orbits of Mars, Earth, and Venus. 
If the sun and the other orbits be covered, no one of 
these can be distinguished from a circle. That point 
of a planet's orbit which is nearest to the sun is its 
'perihelion; 1 the point which is farthest from the sun is 
its aphelion. 2 In Fig. 4, P is the perihelion, and A the 
aphelion. The difference between the distances of 
these two points from the sun may be very consider- 
able, even if the orbit does seem to be almost a circle. 
In the case of the earth the difference is three millions 
of miles, and with most of the other planets the differ- 
ence is greater. Some of the comets are thought to 
move not in ellipses, but in parabolas. 3 The two sides 
of this curve (Fig. 5) keep separating farther and far- 
ther forever. The parabola is not a closed curve like 
the circle and the ellipse. 

26. Characteristics of all the Planets. — Next to the sun 
the planets are the most important parts of the solar 
system. They are alike in many points. Besides 
moving about the sun in the same direction in ellip- 
tical orbits, they all seem to revolve upon their axes in 
the same direction, giving them all day and night. 
Their paths all lie nearly in the same plane. They 
are all of the same shape. They all shine by reflected 
sunlight. 

t 

1 Perihelion, from the Greek peri, near, and helios, the sun. 

2 Aphelion, from the Greek apo, from, and helios, the sun. 

3 The paraVola is so drawn that every point of the curve is equally- 
distant from a fixed point and a fixed straight line. As in Fig. 5, 
CD and CS are equal ; S is the focus, and DD' the directrix. 



GENERAL VIEW OF THE SOLAR SYSTEM. 35 



27. Statistics of the Sun and Planets. 



Name. 


Average 
dist. from 
the sun. 


05 
© 

i 

© 
© 

s 

s 


« 

o 
S3 

"So 

a 
© 


4 

Cm 

O 

to 
a 
© 

Hi 


Mass (times the weight 
of the earth). 


Density (times the 
weight ot water). 


3 
o 

'■s 

"8 

3 

© 
o 
o 
H 


<*- 
o 


4) 5Q 

a. si 


Sun 






866,000 

3000 
7630 
7918 
4200 

20 

to 

300 (?) 
86,000 
73,000 
32,000 
35,000 




25 days. 

88 days ? 
225 days? 
23h. 56m. 
24h. 37m. 

unknown. 

9h. 55m. 
lOh. 14m. 
unknown, 
unknown. 


330,000 

i 

i 

unknown. 

312 
93 
14* 
17 


11 

6| 
4f 

4§ 

unknown. 

If 
I 

li 
IS 




Mercury 


36 

67 

93 

142 

200 
to 
325 
483 

886 
1,782 
2,790 


§ 

1 

1| 

2 
to 
3§ 

5 

9$ 
19 

30 


days. 

88 
225 
365J 
687 
years. 
3 

to 
7 

12 

29i 

84 
164i 


0.2056 
0.0068 
0.0168 
0.0933 

0.02 
to 
0.38 
0.0483 
0.0560 
0.0464 
0.0090 






Planetoids... < 




Uranus 





This table is not to be committed, as the most im- 
portant of these statistics will be given in round num- 
bers in connection with the separate planets, but some 
of its striking facts should be noticed. The sun is by 
far the largest body in the solar system. His mass is 
seven hundred times that of all of the planets together. 
The planets are divided into three groups. Nearest 
the sun are four small planets, not differing very greatly 
in size ; of these the earth is the largest. Next to these 
is a large number of very small planets, or planetoids. 
Then come four giant planets, which in several re- 
spects resemble one another. The four small planets 
are of heavy material ; the sun and the four large 
planets are all about as light as water. Two of the 
four small planets have days about twenty-four hours 
'long, while all of the large planets whose axial rev- 
olutions have been determined have days of only 



36 



ASTRONOMY. 



ten hours. Figs. 6 and 7 will assist in giving clear 
ideas of the sizes and distances of the planets. In 

Fig. 7 two of the 
planetoids are put 
between Mars and 
Jupiter. These 
planetoids are very 
small planets. They 
all come between 
Mars and Jupiter, 
and are close to- 
gether. Little is 
known about them. 
The planetoids and 
the two farthest 
planets, Uranus 
and Neptune, were 
unknown to the 
ancients. 

28. Satellites. — 
All of the principal 
planets except the 
two inner ones, 
Venus and Mer- 
cury, have one or more satellites. The earth has one 
satellite, the moon ; and the satellites of the other 
planets are often called their moons. These satellites 
all revolve around their planets, just as the planets re- 
volve about the sun, and are carried with them by the 
planets in their journey about the sun. 

29. Kepler's Lavis. — As has been said, Kepler discov- 
ered three important laws, by which the motion of all the 
planets and their satellites is interpreted. These are : 




Fig. 6.— The Comparative Size of the Planets. 



GENERAL VIEW OF THE SOLAR SYSTEM. 37 



I. The planets move in ellipses, with the sun in one focus. 

Before the discovery of this law, astronomers had 
always assumed 
that the planets 
move in circles, 
and it must not be 
forgotten that these 
ellipses are almost 
circles. When it is 
at perihelion, or 
nearest the sun, a 
planet moves fast- 
est ; if it did not 7 
the increased at- 
traction of the sun 
would cause the 
planet to fall into 
it. This is be- 
tween P 2 and P 3 in 
Fig. 8 ; but as the 
planet moves from 
P 3 to P 4 , the sun's 
attraction, pulling 

it back makes itS FlG *7- — The Comparative Sizes of the Sun, as seen 
.. from the Different Planets. 

motion slower and 

slower, until between P 4 and P 5 it is slowest of all. If 
this were not the case, the sun's attraction upon it here 
would be too weak to hold it in its place, and it would 
fly off into space. As it turns and passes through P 
and P 1 , the sun's attraction pulls it forward and contin- 
ually increases its velocity, so that at perihelion the 
planet's motion is swift enough to carry it past the sun 
without falling into it. 




38 ASTRONOMY. 



II. The radius-vector of each planet sweeps over equal areas 
in equal times. 

The radius-vector is the line drawn from the sun to 
any point of the orbit, as SP, SP 1 , SP 2 , etc., in Fig. 8. 



/ we 




Fig. 8. — Illustrating Kepler's Second Law. 

In this figure suppose that PP 1 , P 2 P 3 , and P 4 P 5 each 
represent the path of a planet for two weeks. Then 
the three shaded parts will be equal in area. 

III. The squares of the times of revolution of two planets 
are proportional to the cubes of their distances from the sun. 

To illustrate this law, let us compare the times and 
distances of Mercury and Mars, from the table on 
page 35, and by this law we shall have : 

88 2 : 687 2 : : 36,000,000 3 : 141,000,000 3 

/Mercury 's\ /Mars's]\ /Mercury 's\ / Mars's \ 

V period. ) Vperiod./ \ distance. / V^istanceJ 

If this be worked out, the product of the means will 
be found to be nearly equal to the product of the ex- 



GENERAL VIEW OF THE SOLAR SYSTEM. 39 

tremes. 1 The same proportion will , be true for any 
other pair of planets. Observation has fixed the times 
of revolution of the planets very exactly, and when 
the distance of the earth from the sun is found, 2 the 
third law enables us to find the distance of any other 
planet from the sun by the proportion : 

Square of . square of . . cube of . cube of 

earth's period • planet's period • • earth's distance • planet's distance. 

Knowing the first three terms of this proportion, the 
last is found by arithmetic. This is the method used 
by astronomers to find the distances of the planets. 

It also follows from this law that the planets near 
the sun move much faster than the distant ones. 
The table shows that Neptune is eighty times as far 
from the sun as Mercury, and its orbit is then eighty 
times as long. But it takes Neptune seven hundred 
times as long to complete its circuit. Mercury must 
move nearly nine times as fast as Neptune. Every 
planet moves faster than the planets outside of it. If 
it did not, it could not keep from being pulled in to- 
wards the sun by his greater attraction. 

30. Ecliptic. — As has been said, the earth revolves 
about the sun once a year, but, as in all such cases, it 
seems to us that the earth is stationary, and that the 
sun moves about it. The apparent yearly path of the 



1 The products will not be found to agree exactly, chiefly because 
the distances and times used above are not quite exact. If the exact 
distances and times were used, the agreement would be still a little 
imperfect, because the different planets influence the motions of one 
another slightly. 

2 The method of finding the distance from the earth to the sun will 
be explained in chapter iii. 



40 



ASTRONOMY. 



sun among the stars is called the ecliptic} The earth's 
axis is not perpendicular to the plane in which the sun 
moves, but is inclined to it. The angle between the 
ecliptic and the equator is 23J degrees. Fig. 9 shows 
this leaning of the earth's axis. SS' is the ecliptic. 




Fig. 9.— The Ecliptic. 

EQ is the equator. The plane of the ecliptic cuts the 
earth along TT'. The angle EVT is the angle of 
23J degrees. When the sun is at S it is directly over 
T, which is 23J degrees south of the equator. This is 
the winter solstice ; 2 it comes on the 21st of December. 
This is the shortest day of the year, the sun being far- 
thest south. As the sun moves around from S towards 
S' it shines directly down upon the line TVT', and is 
getting farther north, nearer the equator. On the 20th 
of March the sun is half-way from S to S', and then 
shines directly down upon the equator at V. This is the 
vernal equinox? or spring equinox. On the 21st of June 

1 So named because eclipses can occur only when the moon is near 
this line. 

2 Sol'stice, from the Latin words sol, the sun, and sto, to stand, 
because the sun seems to stand still here a short time before turning 
to the nortff. 

3 E'qui-nox., from the Latin words equus, equal, and nox, night, 
because the nights and days are here equal. Yernal, from the Latin 
adjective vernalis, spring. 

The dates of the solstices and equinoxes may vary a day, because 
365 or 366 days do not make an exact year. 



GENERAL VIEW OF THE SOLAR SYSTEM. 41 



the sun is at S', the summer solstice, and is now farthest 
north, being directly over T'. Half-way from S 7 to S, 
on September 22, the sun again crosses the equator, 
giving us the autumnal equinox. 

The student must again carefully distinguish this 
motion of the sun among the stars from its apparent 
daily motion from east to west. If the stars could be 
seen in daytime, the sun would be seen to be slowly 
moving among them towards the east, just as the moon 
does at night ; it is this path that is the ecliptic. 

The ecliptic is divided into twelve equal arcs of 30 
degrees each, called signs. They begin at the vernal 
equinox, and take their names from the names of twelve 
constellatious, or groups of stars. Their names — which 
are all Latin — and symbols are these : 



A'ri-es, <Y> (the ram) 
Tau'rus, & (the bull) 
Gem'i-nt, n (the twins) 
Cancer, qz> (the crab) 
Le'o, % (the lion) . 
Yir'go, TTJj (the virgin) 
Li / bra, =£= (the balance) 
Scorpio, rr^ (the scorpion) 
Sagittarius, / (the archer) 
Capricor^us, l/j (the goat) 
Aqtta'riits, c# (the waterman) 
Pisces, )£ (the fishes) 



From the 
Vernal Equinox. 

0° to 30° 

30° to 60° 

60° to 90° 

90° to 120° 

120° to 150° 

150° to 180° 

180° to 210° 

210° to 240° 

240° to 270° 

270° to 300° 

300° to 330° 

330° to 360° 



The sun enters the sign Aries at the time of the ver- 
nal equinox, about March 20, and about a month later 
enters the second sign, Taurus, and so on through 
them all during the year. These signs and their sym- 
bols are in the first part of our common almanacs. 

31. The Celestial Equator. — The celestial equator is a 

4* 



42 ASTRONOMY. 



great circle around the heavens, right above the equa- 
tor on the earth. It cuts the ecliptic at the equinoxes, 
making an angle with it, of course, of 23 J degrees. If 
the equator were visible in the sky, it would appear as 
an arch, passing across our southern sky, cutting the 
horizon just east and west of us. The path of the sun 
on March 20, or September 22, is on the equator. In 
summer the sun's path is higher up in the sky than the 
equator ; in winter it is lower. 

Latitude and longitude are used to fix the position 
of places on the earth, and in the same way places in 
the sky are located; but, unfortunately, astronomers 
use other names than latitude and longitude to indicate 
corresponding distances. The distance of a star north 
or south of the equator is called its declination. In- 
stead of the meridian of Greenwich or Washington to 
reckon longitude from, the meridian passing through 
the vernal equinox is used. And the distance that a 
star is east of the vernal equinox is its right ascension} 
Both declination and right ascension, like latitude and 
longitude, are reckoned by degrees. 

1 Declination, like latitude, is measured both north and south from 
the equator to the poles, but right ascension is measured around by 
the east only. So that a heavenly body may have any right ascen- 
sion up to 360°. 

What is the greatest possible declination that any point can have ? 
where is that point ? What is the dec. of the sun on June 21 ? on 
September 22 ? What is the dec. of your zenith ? (See Fig. 2). 

What is the R. A. (right ascension) of the sun on March 20? on 
June 21 ? on December 21 ? In which sign is the sun when his R. A. 
is 50° ? when it is 140° ? 250° ? When the sun's R. A. is 110° is its dec. 
north or south ? when its R. A. is 180° ? when it is 300° ? What is the 
sun's dec. when its R. A. is 90° ? when 180°? when 270° ? when 360° ? 

Right ascension is usually reckoned in hours hj astronomers, 1 
hour being 15 degrees. 



GENERAL VIEW OF THE SOLAR SYSTEM. 43 

32. The Zodiac. 1 — The zone of the heavens, extending 
about eight degrees on each side of the ecliptic, is called 
the zo'diac. It too is divided into twelve signs, which 
have the same names and order as the signs of the 
ecliptic. These signs roughly coincide with twelve 
constellations, or groups of stars, and it was to these 
constellations that the ancients gave the names Aries, 
Taurus, etc. When these names were given, the sun 
entered the constellation Aries at the time of the ver- 
nal equinox, and the signs of the ecliptic, through 
which the sun moves, coincided with the constellations 
marking the signs of the zodiac. But the vernal equi- 
nox, the point where the sun crosses the equator in the 
spring, moves very slowly backward, so that now the 
sun comes to the vernal equinox about a month before 
it enters the constellation Aries. The sun, therefore, 
is in the sign Aries while it is in the constellation Pisces, 
and in the sign Taurus while in the constellation Aries, 
etc. The signs of the ecliptic are about one place ahead 
of the corresponding signs and constellations of the 
zodiac. 

Although the planets all move about the sun in the 
same direction, yet their orbits do not lie in the same 
plane. But the angles winch the planes of the orbits 
make with each other are all small, and the planets are 
always found within the zodiac. Their paths are ap- 
parently circles, cutting the ecliptic at two points 180 
degrees apart. These points are called nodes. Since 
the planets are always so close to the ecliptic, when- 
ever they can be seen they show us just about where 
the ecliptic lies in the sky. 

1 From the Latin Zobn, an animal. So named from the animals 
with which the ancients supposed it peopled. See page 4>. 



44 



ASTRONOMY. 



CHAPTER II 



THE SUN. 



Distance from the Earth, 93,000,000 Miles. 1 Diameter, 
866,000 Miles. Axial Rotation, 25 Days. Specific Grav- 
ity, 1.4. 

33. The Sun's Parallax. — In finding the distance from 
the sun to the earth, astronomers have generally tried 
to determine first the sun's parallax. 2. The parallax of 
a heavenly body is the angle that the earth's radius 
would make if seen from that body. And so the sun's 
parallax is the angle that the earth's radius of nearly 
four thousand miles would make, or, more properlj 
speaking, would subtend, if looked at from the sun. 




Fig. 10.— The Sun's Parallax (greatly exaggerated). 

Fig. 10 will make this clear. E is the centre of the 
earth, and AE is the earth's radius. Then, if S repre- 



1 In kilometres, now so frequently used for scientific measurements, 
the sun's distance is between 149 and 150 millions. A kilometre is 
nearly two-thirds of a mile. 

2 Par'al-lax, from a Greek word spelled almost exactly the same 
way, and having the same meaning. 



THE SUN. 45 



sents the sun, the angle ASE is the sun's parallax. 1 An 
accurate measurement of the sun's parallax is exceed- 
ingly difficult, but so great is its importance that many 
efforts have been made to determine it. Some of the 
most successful methods will be explained later in the 
book, Arts. 56, 57. It is a very small angle; the 
best measurement so far makes it 8.81". 2 The angle 
at S in Fig. 10 is greatly exaggerated; it is almost 
three thousand times as large as the real angle. To 
represent it exactly in a figure is of course impossible. 
It is the angle which a foot-rule would subtend at a 
distance of four and a half miles. 

34. Distance and Size of the Sun. — Since the earth's 
radius is known very exactly (Art. 65), when we know 
the angle that it subtends at the sun, it is an easy 
problem in trigonometry to calculate the distance 
of the sun, 3 which will be found to be a little less than 
93,000,000 miles. This distance has been aptly called 
the yard-stick of the universe. Our measurements of 
the distances and dimensions of all the other planets, 



1 Properly speaking, this is the horizontal parallax, — that is, the 
angle subtended by the radius running to our feet when the sun is on 
the horizon. It is easily seen that if the sun were above its position 
in Fig. 10, the angle ASE would be smaller. And if the sun were 
directly above A, this angle would be zero. 

2 A few of our readers may need to be reminded that this is 8.81 
seconds, and is angular measure. It must not be confounded with 
seconds of time, which are never indicated by these two strokes /7 , 
but always by s, or sec. 

3 The following proportion will make this clear to those who under- 
stand trigonometry. Using the triangle in Fig. 10, in which A is a 
right angle and ASE is the parallax, we have : 

Sin parallax : sin 90° : : earth's radius : dist. from sun to earth ] 
or, sin 8.81 7/ : sin 90° : : 3959 : required distance. 



46 ASTRONOMY. 



and even of the distances of the fixed stars, depend 
upon it. If the distance to the sun is determined 
more accurately, all these distances and dimensions as 
given in this book should be proportionately changed. 
On this account these figures will be found to differ 
in different astronomies. 

By measuring the apparent angular diameter 1 of the 
sun, and knowing its distance from us, another simple 
trigonometrical solution gives us its diameter, 2 which 
is about 109 times the earth's diameter. And since 
the volumes of spheres are as the cubes of their diam- 
eters, the sun's volume is 109 3 , or about 1,300,000 times 
that of the earth. But the density of the sun is only 
about one-fourth of the earth's density, so that while 
it would take 1,300,000 worlds as large as ours to make 
one as large as the sun, yet it would only take one-fourth 
of this number, or about 325,000, to make one as heavy 
as the sun. The force of gravity upon the sun is much 
greater than upon the earth, and, as the weight of 
a body depends upon gravity, anything would weigh 
nearly twenty-eight times as much upon the sun as upon 
the earth. A man who weighs one hundred and fifty 
pounds here would weigh more than two tons upon 
the sun, and would be crushed to death by his own 
weight. 

The Sun and his Surroundings. 

35. The Sun's Outer Atmosphere. — If it were possible 

1 The angular diameter of the sun is the angle which its diameter 
subtends as seen from the earth. 

2 If a right-angled triangle be drawn, having the line from the 
centre of the earth to the centre of the sun as its hypothenuse, and its 
right angle at the surface of the sun (because the line along which 
the edge of the sun is seen is a tangent), we have : 

Sin 90° : sin of half of sun's angle : : 93,000,000 : sun's radius. 



THE SUN. 47 



to visit the sun, one would first enter the corona? a 
very light atmosphere extending several hundred thou- 
sands of miles on all sides. It is never seen except 
during a total eclipse, and then is a bright cloud-like 
circle of light surrounding the darkened sun. A great 
part of the corona is made up of streamers of light 
extending from the sun in various directions. Some- 
times these streamers stretch away in tw T o opposite 
directions only; often they project in four directions, 
giving the corona a four-sided appearance. At the 
eclipse of 1878 these streamers were noticed by some 
observers to extend as far as 9,000,000 of miles from 
the sun. The corona is never twice of the same shape, 
and even during the same eclipse its shape appears 
very different to different observers. 2 Photographs of 
it taken from different points on the earth at about 
the same time will, however, show the same general 
features. Fig. 11 represents a sketch of the corona as 
seen by Prof. Stone 3 during the eclipse of 1878. 

The spectroscope (Art. 254) shows that the corona 
is composed mostly of hydrogen, which is the lightest 
known gas upon the earth, and some unknown gas or 
vapor even lighter than hydrogen, which has been 

1 Cor-6'na, Latin corona, a crown. 

2 This is remarkable. Different observers of the same eclipse, even 
when sitting side by side, make totally different drawings of the same 
corona. This is probably because one observer's attention is attracted 
mainly or even only by those features of the corona which strike him 
as most prominent,— perhaps the great length or breadth of certain 
streamers. Another might notice particularly, and therefore draw 
only, the brighter parts of the corona. And owing to the short time 
that the eclipse lasts and to the excitement of the observers, probably 
none of them will notice all the parts of the corona. 

3 Ormond Stone, 1847 — , director of the Observatory of the Univer- 
sity of Virginia. 



48 



ASTRONOMY. 



called coronium. These gases give out light of them- 
selves, and not merely reflect the sunlight. They are 
exceedingly thin and rare, more so than our own atmo- 



: vjbw 

'•7:7 /./'v_5:^ 
xmkmmr 



N 



=^s 




W 



Fig. 11. — The Corona as sei.n in 1878. 



sphere, and the streamers are probably more like the 
streaks of "Northern Lights" than anything else we 
know of on the earth. They also have a certain re- 
semblance to the tails of comets, and may owe their 
origin to electrical action. 

36. The Sun's Lower Atmosphere. — The lower part of 
the sun's atmosphere, which rests directly upon the 
sun itself, is called the chromosphere} It is a sheet of 
flame several thousands of miles deep surrounding the 
sun. The spectroscope shows that the chromosphere is made 
up of the burning vapors of iron, copper, sodium, and some 

1 Chro'mo-sphere, from the Greek chroma, color, and sphere. It 
is this layer of burning vapors that causes the dark lines in the sun's 
spectrum, as is explained in Art. 253. 



THE SUN. 



49 



twenty or more other substances which we find upon the earth. 
Besides these, there are several substances burning 
in the chromosphere which have never been found 
upon the earth. This discovery of the substances 
which compose the chromosphere is one of the most 
remarkable of modern times. It was made by Prof. 
G. K. Kirchoff (kirk'hof ), of Germany, in 1859. The 
chromosphere cannot be seen with the naked eye, nor 
with an ordinary telescope, except during a total eclipse 
of the sun. But by having a spectroscope attached to a 
telescope (Art. 254), and directing it to the edge of the 
sun, the chromosphere can be observed on any clear day. 
37. The Solar Prominences! — Terrible storms are con- 
stantly raging in the chromosphere. From every part 




Fig. 12. — Changes in a Sun Prominence during ten Minutes, Observed by Professor 
Young, October. 7, 1869. 

of the sun's surface great masses of the burning vapors 
are frequently hurled up to a height which not uncom- 
monly reaches 100,000 miles. Prof. Young, in 1880, 

5 



50 ASTRONOMY. 



saw one thrown up to the enormous height of 350,000 
miles. These are the red prominences seen during 
total eclipses of the sun, and now, with the aid of the 
spectroscope, watched every day. These masses are 
frequently thrown up with a velocity of 100 miles, and 
sometimes even 200 miles, per second. They are largely 
composed of burning or glowing hydrogen, but some- 
times, near the base, of the burning vapors of the 
metals and heavy elements which make up the sun. 
They must be caused by great eruptions or explosions 
in the sun or the chromosphere. Fig. 12 shows the 
sudden changes in one of these prominences, as seen 
by Prof. Young 1 in 1869. Others of the prominences 
remain unchanged in form and position for days. 
These may be great masses of clouds thrown up by 
an explosion, which remain floating in the sun's at- 
mosphere. 

38. The Surface and Interior of the Sun.— Below the 
corona and chromosphere we come to the. surface of 
the sun itself, the only part of it ever seen by most 
people, called by astronomers the photosphere. 2 This is 
now generally believed to be a shell of clouds 'surround- 
ing the unseen mass of the sun* beneath. Every one 
knows that the clouds about the earth are made up of 
tiny drops of water, that clouds are in fact precisely 
like fogs, except that they are floating high up in the 
air. The clouds which make up the sun's surface are 
not composed of water, but of tiny drops of fiery-hot 
melted iron, copper, and other substances that consti- 
tute the chromosphere. 

1 Charles A. Young, 1834 — , Professor of Astronomy at Princeton, 
New Jersey. 
* Pho'to-sphere, from Greek phos, light, and sphere. 



THE SUN. 



51 



Within the photosphere is the body of the sun, and, 
strange- as it may seem, it is now generally believed 
that this is a great ball of gas ; in fact, an enormous 
bubble. The great pressure makes this gas denser 
than water, so that it is not light and thin like the air 
around us, but probably as thick as tar or jelly. This 
gas is no doubt composed of the vapors of the various 
substances which make up the chromosphere. These 
are all kept in the condition of vapor by the intense 
heat. 

39. Sun-spots. — With a small telescope the only thing 
to be seen on the sun's surface is a greater or less 




Fig. 13. — Sun-spots and Facul^:. (From Young's The Sun.) 

number of dark spots. The shapes of these are very 
various and irregular. The central part of a spot, 
called the nucleus, or umbra, 1 is black, while around the 



1 Um'bra, Latin umbra, a shadow. 



52 ASTRONOMY. 



edge is a lighter, grayish border, the penumbra} Fig. 
14, a drawing of a sun-spot seen through a large, tele- 
scope in 1860, shows very clearly the features of a 
sun-spot. Here filaments of the penumbra stretch 
entirely across the umbra ; but this is unusual. These 
spots are of all sizes, from those just visible in large 
telescopes to occasional monstrous ones 100,000 miles 
in diameter. They are very commonly found in groups, 
and are not distributed over the whole surface of the 
sun, but are confined to two zones, one on each side 
of the equator. These zones begin about 10° from the 
equator, and end about 35° from it. Close to the sun's 
equator spots are rarely seen, and close to the poles, 
never. As the sun turns upon its axis, the spots are 
carried along with it, and so pass across the sun's 
disk in twelve or fourteen days ; it is by the motion 
of the spots that we can tell that the sun rotates, and 
determine the time of its rotation. Besides being thus 
carried around by the sun, the spots have some motion 
of their own over the sun's surface. Careful observa- 
tions have shown also that the spots in different lati- 
tudes have different rates of rotation. Spots on the 
equator revolve in twenty-five days, those farthest from 
the equator in twenty-six or twenty-seven days. This 
remarkable fact has made it very difficult to decide 
what the period of the sun's rotation really is, but, as 
Prof. Young says, " the probability is that the sun, 
not being solid, has really no exact period of rotation, 
but different portions of its surface and of its internal 
mass move at different rates, and to some extent inde- 
pendently of each other." 

1 Pe-num'bra, Latin pene, almost, and umbra, shadow. 



THE SUN. 



53 




Fig. 14. — A Group of Sun-spots. (From Young's The Sun.) 

5* 



54 ASTRONOMY. 



40. Phenomena and Cause of Sun-spots. — The spots are 
certainly great cavities in the surface of the sun, the 
bottom of the cavity forming the umbra, and the sides 




Fig. 15.— The Changes in the Appearance of a Sun-spot as it is carried 
across the Sun's Disk by the Rotation of the Sun. (From Newcomb's Pop- 
ular Astronomy.) 

the penumbra. This is shown by the appearance of a 
spot as it is first brought into view by the revolution 
of the sun. This may be seen in Fig. 15. When the 
spot is first seen on the edge of the sun, the penumbra 
and side of the umbra nearest to us would be hidden, 



THE SUN 55 



but as the sun turned the whole spot would pres- 
ently he seen. In going off the sun, the other side is 
hidden first. That the sun's spots are cavities is also 
conclusively proved by the fact that when just upon the 
edo-e of the sun they have sometimes been seen to be 
notches. The umbra is not entirely dark, but only so 
much darker than the brilliant surface of the phato- 
sphere as to look dark when compared with it. The 
highest artificial light that can be made, except the 
electric light, seems absolutely black compared with the 
sun's light. Spots may last for months, or only for 
hours. They appear and disappear with great rapidity, 
and frequently change their size and appearance greatly 
from day to day. A large spot frequently breaks up 
into small ones, and a group of small ones as frequently 
combine to make a large one. 

The cause of the sun-spots is another of the mysteries 
of this wonderful body. As has been said, they are 
certainly great hollows or cavities, and may be caused 
by great whirlwinds, just as we see whirlpools formed 
in water. And there is evidence that these cavities are 
filled with gases and vapors, which obstruct the light 
from below, and so cause the dark parts of the spot; 
that they are places where the gases, which have been 
forced up in the prominences, and cooled in the upper 
layers of the atmosphere, are again drawn down into 
the sun. 

41. How to Observe the Sun-spots. — When the spots 
are very large, they can be seen by the naked eye, 
looking, of course, through smoked or colored glass ; 
but this is uncommon. With any good spy-glass they 
can generally be seen. If they are observed directly 
through a spy-glass or a telescope, the eye-piece must 



56 ASTRONOMY. 



always be covered with a dark glass. Several astrono- 
mers have lost their eyesight by looking at the sun 
through a telescope without using the colored shade. 
Unless a small glass, or very low magnifying power, is 
used, one must not expect to see the whole sun at 
once ; the instrument must be moved gently over the 
surface to scan it all. It may be well, too, to remind 
the young observer that an astronomical telescope al- 
ways inverts the object seen : what seems in the tele- 
scope to be the louver part of the sun is really the upper 
part, what seems to be the right side is the left side. 
But a spy-glass does not invert the object. 1 

A better way to observe the sun-spots is to throw 
the sun's image upon a screen with the spy-glass or 
telescope. For this a room having a window towards 
the sun must be chosen, and it must be darkened with 
shutters or curtains. The instrument is then pointed 
through a hole in the shutter or curtain at the sun, just 
as if it was to be observed in the ordinary way. But 
instead of looking at the sun, place a screen or piece 
of white paper perpendicular to the telescope and a 
short distance, say a foot, from the eye-piece, when a 
brilliant image of the sun will be seen upon the screen. 
The instrument ought to be upon a stand, or supported 
by some fixture attached to the window or shutter, and 
may be directed to the sun by glancing along the top 
of the tube. "When the image is once thrown upon 
the screen, it can easily be kept there by gently moving 
the telescope as the image passes off. The instrument 
must be focused by moving the eye-piece in or out 
until the picture is most distinct. The whole of the 
sun will not generally be shown at once, but by moving 

1 The cause of this is explained in Art. 244. 



THE SUN. 57 



the instrument all the different parts of its surface may 
be thrown upon the screen and examined. No dark 
glass is needed to cover the eye-piece, and the sun's 
image with its spots may be seen by a number of per- 
sons at once. The image may be made as large as is 
wished by moving the screen farther off, or throwing 
the picture upon the opposite wall, but the smaller 
images will be most brilliant. By this method all the 
phenomena and changes of the spots may be carefully 
studied, their motions and changes noted, and their 
outlines drawn upon the screen itself. If an astro- 
nomical telescope is used, the observer will not forget 
that the motions of the spots are just opposite to their 
apparent motions from day to day upon his screen, and 
to give them their correct positions his drawings must 
be turned upside down. The general motion of the 
spots is from east to west, or, when looking at the 
sun, from left to right, but not directly across; the 
direction of the motion shows that the sun's axis is 
inclined to the plane of the earth's orbit. No heavenly 
body can be observed or studied with more interest by 
the owner of a spy-glass or small telescope than the sun. 
Accurate and complete records of sun-spots, accom- 
panied if possible by drawings, would be valuable con- 
tributions to science, and many important discoveries 
have been made in this field with small instruments. 

42. Periodicity of the Spots. — Long observations have 
proved the curious fact that sun-spots are most abun- 
dant about every eleven years. In 1848, 1860, and 
1870 they were most numerous, while in 1856, 
1867, and 1878 they were fewest. In 1882 and 1883 
they were abundant, especially in the latter year; 
then they diminished in size and frequency until 



58 ASTRONOMY. 



1889, increasing again until the next maximum in 
1893. No satisfactory cause of this periodicity has 
been discovered. Observation has also shown a con- 
nection between the sun-spots and magnetic disturb- 
ances upon the earth. When sun-spots are most 
abundant, magnetic storms are most frequent; that is, 
compass-needles are turned from their proper direction, 
strong magnetic currents take possession of the tele- 
graph wires, interfere with the sending of messages, 
and even set telegraph-offices on fire. These magnetic 
storms may be noticed over the whole earth, and are 
sometimes accompanied by unusual displays of the 
aurora, or northern lights. Like the sun-spots, these 
phenomena are periodical, and their periods coincide 
with the sun-spot periods. The cause of this coinci- 
dence is unknown, but there is probably an electrical 
connection between the sun and the earth. All of 
these phenomena are well worth observing and noting 
down. Such observations may lead to valuable results. 

43. Other Markings on the Sun. — Seen through a good 
telescope, the whole bright surface of the sun is mot- 
tled, being covered apparently by bodies which from 
their shape and appearance have been called rice-grains. 
"Perhaps the most familiar idea of this appearance 
will be presented by saying that the sun looks like a 
plate of rice-soup, the grains of rice, however, being 
really hundreds of miles in length." 1 Under very 
favorable circumstances these rice -grains have been 
seen to be made up of smaller granules. 

About twenty years ago, Mr. Nasmyth, an English 
astronomer, announced the discovery that through a 

1 Newcomb's Popular Astronomy, p. 237. 



THE SUN. 59 



powerful telescope this mottled appearance of the sun 
was seen to be due to a mass of long narrow bodies 
intertwined into a complete net-work over the whole 
surface of the sun. These did not seem to him to be 
shaped like rice-grains, but like willow-leaves. These 
willow-leaves, as they were called, have not been seen 
bv other observers, and their existence is doubtful. 
Fig. 14 was drawn by Mr. Xasmyth, and shows around 
the spot the appearance resembling willow-leaves that 
he thought he saw. 

Bright streaks are often seen upon the sun, some- 
times separate, and sometimes forming a net-work. 
These are called faculce} They are temporary ridges 
on the surface of the sun ; this is proved by the fact 
that they have been seen to project above the edge of 
the sun. They are sometimes many thousands of miles 
long. They are abundant about the edges of the sun- 
spots, and, like the spots, they are constantly appearing, 
disappearing, and changing their forms. In years 
when sun-spots are few faculse are few also. They 
seem to be heaped up by the great storms and other 
commotions on the sun, especially when a sun-spot is 
formed or disappears. The white cloud-like patches 
shown in Fig. 13 are the faculee. The faculse, as well 
as the rest of the sun, are made up of the rice-grains. 
These faculse can be seen with a telescope of mod- 
erate size, and may be observed directly or thrown 
upon a screen as the sun-spots were. They should be 
looked for around spots which are near the edge of 
the sun. The mottled appearance of the sun may be 
seen in the same ways, but needs at least a good 

1 Fac'u-lae, plural of Latin /acuta, a torch. 



60 ASTRONOMY. 



three-inch telescope 1 and careful observation. The 
separate rice-grains can be seen only through large 
telescopes. 

44. The Sun's Position and Importance in the Solar 
System. — The sun is the centre of the solar system, and 
his mass is 700 times as great as that of all the other 
bodies in the system put together. On account of his 
overwhelming size, his great attraction controls the mo- 
tions of all the planets, and keeps them in their orbits. 
"Were this attractive force of the sun to cease, the 
whole system would at once go to pieces, the planets 
would fly off into boundless space, and all life upon 
the earth or elsewhere in the system would speedily be 
destroyed. 

45. The Sun's Light Its Amount and Importance. — 
The sun's light is the most intense light known to us. 
It is from three to four times as bright as the brightest 
electric light; and every other artificial light seems 
absolutely black when put in front of the sun. Several 
attempts have been made to measure the brightness of 
the sun's light. This can be done only by comparing its 
light with some other light. For instance, it has been 
found that the sun gives out 600,000 times as much 
light as the full moon, while the light of the full 
moon is about the same as that of a candle twelve 
feet away. Much of the sun's light is absorbed by the 
atmosphere of the sun, and some by the atmosphere of 
of the earth. Were these removed, Professor Langley 
has calculated that the sun would be two or three times 
as bright as now, and blue instead of yellow. 



1 The size of a telescope is generally designated by the diameter of 
its object-glass. 



THE SUN. 61 



Of the importance of the sun's light it is scarcely 
necessary to speak. Without it we should have only 
starlight in addition to our artificial light ; for the 
moon shines wholly by reflected sunlight. Besides its 
great importance in vision, sunlight is essential to 
vegetable life, and indirectly, therefore, if not directly, 
to all animal life. 

46. The Sun's Heat — Its Amount and Importance. — 
The amount of heat which the sun gives out is beyond 
all our conception. That which the earth receives 
every year would melt a shell of ice about 165 
feet deep covering the whole earth. Yet this is 
only 2,300,000.000 l of all that the sun sends out. As 
Proctor puts it, " In each second the sun gives out 
as much heat as would be given out by the burning 
of 11,000,000,000,000,000 tons of coal." 

Without the sun's heat the temperature of the earth 
would be some hundreds of degrees below zero, a tem- 
perature at which it would be impossible for life to 
exist. But this is not all, for to the sun's heat almost 
all motions on the earth are due. All the winds, all 
the clouds and storms, and consequently all springs and 
rivers, are due to the sun's heat. All our wood and 
coal represent just so much of the sun's heat stored up 
in the past. And, since it is now known that one sort 
of force may be changed into another, the sun's heat 
must be considered the real cause of almost all the 
forces, of all the work, and of all the power in the 
world. The tides are perhaps the only exception, for 
they are due mainly to the moon's attraction. But it 
is the sun's heat alone that keeps the water in a liquid 

1 How is this calculated ? 
6 



62 ASTRONOMY. 



state, and thus allows it to form tides. Tyndall well 
says, " The natural philosopher of to-day may dwell 
amid conceptions which beggar those of Milton. Look 
at the integrated energies of our world, — the stored 
power of our coal-fields; our winds and rivers; our 
fleets, armies, and guns. What are they ? They are 
all generated by a portion of the sun's energy, which 
does not amount to 2,300,000,000 of the whole. This is 
the entire fraction of the sun's force intercepted by 
the earth, and we convert but a small fraction of this 
fraction into mechanical energy. Multiplying all our 
powers by millions of millions, we do not reach the 
sun's expenditure. And still, notwithstanding this 
enormous drain, in the lapse of human history we are 
unable to detect a diminution of his store. Measured 
by our largest terrestrial standards, such a reservoir of 
power is infinite ; but it is our privilege to rise above 
these standards, and to regard the sun himself as a 
mere speck in its finite extension, a mere drop in the 
universal sea." 

47. The Cause of the Sun's Heat — The amount of heat 
given off from the sun continually is so enormous that, 
as none of this comes back, it has been a great problem 
to account for this constant supply of heat. We know 
that if the whole sun were a mass of solid coal, it would 
burn out at its present rate in five thousand years ; and 
yet the sun has lasted much longer than that, and, so 
far as we can notice, his heat is not diminishing a par- 
ticle. Two theories have been advanced to account 
for this. One is the meteoric theory. As will be fully 
explained in chapter VIII, it is known that immense 
numbers of small bodies are revolving about the sun, 
and some of these must be continually falling into it, 






THE SUN. 63 



just as they fall upon the earth and give us our shoot- 
ing-stars, but the number there must be vastly greater 
than here. Now, when one body strikes another, heat 
is always produced, as when a nail is struck with a 
hammer. If the striking body moves very swiftly, the 
heat produced is very great. If a combustible body 
were to fall from the earth to the sun, its striking 
would produce 6000 times as much heat as the burn- 
ing of the same body could. And so it has been 
thought that the sun's heat is kept up by the striking 
of these bodies, called meteors, upon its surface. But 
when astronomers came to realize the prodigious heat 
of the sun, they saw that although this cause helps, yet 
alone it could not be sufficient to supply the sun with 
heat. The other theory of the sun's heat is the contrac- 
tion theory. It supposes that by its own attraction the 
sun is slowly contracting in bulk: this condensation 
or squeezing together would produce heat just as a 
body falling upon it would. It has been estimated 
that if the sun's diameter should shorten only six 
miles in one hundred years, as much heat would be 
produced as the sun gives out in that time. No such 
contraction has ever been noticed in the sun, but this 
is no reason why the theory may not be true, for if the 
shrinking has occurred, we could not possibly detect it 
yet. This is the only cause ever suggested that, so far 
as we now know, can be the true one ; and, although it 
has not been proved, it is generally regarded by as- 
tronomers as the principal cause of the sun's heat. 

48. The Sun's Past and Future. — There has been of 
late much speculation upon the probable length of time 
that the sun has existed, and when he will probably 
cease to give out heat. No matter what may be the 



64 ASTRONOMY. 



source of the sun's heat, we are forced to conclude that, 
if natural laws alone operate, his heat must at last be 
exhausted. As the sun gradually cooled off, the earth 
would become colder and colder ; and when all heat 
from the sun ceased, the temperature of the earth 
would, as has been said, probably be hundreds of de- 
grees below zero. Long before this time all life would 
of course perish from the earth. But in any event 
these conjectures need give us little immediate con- 
cern, for the rashest speculators place these events 
millions of vears in the future. 



THE INFERIOR PLANETS. 65 



CHAPTER III. 



THE INFERIOR PLANETS. 



49. Suspected Vulcan. — For some years certain as- 
tronomers have strongly suspected that between Mercury 
and the sun there is a planet, which they have named 
Vulcan. 1 The great French astronomer Le Verrier, 2 
of whom we shall hear in connection with the discov- 
ery of Neptune, found certain irregularities in Mer- 
cury's motion, which he suggested might be caused 
by the attraction of such an inside planet. Observers 
have several times announced that they saw the planet 
crossing the sun's disk; but few, if any, such obser- 
vations have been reported by skilled astronomers, 
and an unpractised observer might easily mistake a 
sun-spot for a planet. Besides, at the very times when 
some of these supposed observations were made, other 
and better observers were also watching the sun, and 
saw no planet. But the strongest evidence in favor of 
Vulcan's existence was given in 1878. During a total 
eclipse of the sun in that year, two American astrono- 
mers, Prof. Watson 3 and Mr. Swift, 4 claimed to have 
discovered two or more planets within Mercury's orbit. 

1 Vul'can, the god of fire. 

2 Le Ver'ri-er, 1811-1877. A great French astronomer; the dis- 
coverer of the planet Neptune. See Art. 179. 

3 James C. "Watson, Professor of Astronomy at Michigan Univer- 
sity, and at University of Wisconsin, 1838-1880. 

4 Lewis Swift, Astronomer of Eochester, New York. 

6* 



66 ASTRONOMY, 



Notwithstanding Prof. Watson's great reputation, as- 
tronomers generally seem to think that he mistook 
small stars for planets. No one has been able to find 
these planets since, though total eclipses have given 
good opportunities, and the existence of Vulcan must 
be regarded as very doubtful. 

i 

MERCUKY. $ 

Distance from Sun, 36,000,000 Miles. Diameter, 3000 Miles. 
Length of Year, 3 Months. Length of Day, Uncertain. Spe- 
. cific Gravity, 7. 

50. Relations to the Solar System, and Features. — So far 
as is certainly known, Mercury 1 is the nearest planet 
to the sun. Seen from him the sun seems seven times 
as large as seen from the earth, and upon his surface 
one would get seven times as much light and heat as 
upon the earth. Mercury is the smallest of the eight 
principal planets ; his volume is only -^ of that of the 
earth. 2 But he is the densest of all the planets, being 
about as heavy as cast iron. The planet is so close to 
the sun that observation of it is very unsatisfactory. 
In the largest telescopes its surface is brilliant, but 
markings are seen with greatest difficulty. Schiaparelli 
of Milan, whose keenness of sight has often been 
proved, thinks he has seen them, and the evidence 
seems to be that the planet rotates on its axis in the 
same time that it revolves around the sun. This would 
keep the same face continually turned towards the sun. 
We shall presently see that the moon moves about 
the earth in this way. Nor is it certainly known 

1 The Latin name of the god who acted as messenger for the other 
gods. The sign represents his rod. All of the principal planets 
except the earth are named for the Latin deities. 

2 How is this found ? 



THE INFERIOR PLANETS. 67 

whether the planet has an atmosphere or not; but it 
is supposed to have a very dense one. 

51. Motions and Phases. — As Mercury is an inferior 
planet, while revolving about the sun it seems to us only 
to swing backward and forward past the sun (Art. 24), 
and is never more than 29° from it. When opposite the 
sun from the earth, it is obscured by the sun's brightness 
and cannot be seen. As soon as it is far enough from 
the sun to be visible, it is small and nearly round. At 
one side of the sun, or at its greatest elongation (Art. 
24), as at E in Fig. 16, it is larger, but only half full. 

Fig. 16. — The Changes in Mercury as it revolves around the Sun. 



As it comes around between the sun and the earth it 
grows still larger, but is a crescent, growing narrower 
and narrower, until in passing between us and the sun 
it is lost in the sun's glare, unless it should happen 
to go directly across the sun, when it could be seen 
as a black spot on his face. 1 These varying phases 
are just like those of the moon, and prove that the 
planet shines by reflecting the sun's light. "When 

1 How far are Mercury and the earth apart when they are on oppo- 
site sides of the sun ? when on the same side ? 



68 ASTRONOMY. 



on the opposite side of the suu, the half of the planet 
lighted up is turned towards us, and it is about full. 
When the planet is at one side of the sun, we see only 
half of the lighted hemisphere, and as it conies more 
and more between us and the sun, its bright side, 
always being the one towards the sun, is turned more 
and more away from us, while the dark hemisphere 
is turned more and more towards us, so that only a 
crescent of light can be seen, growing constantly nar- 
rower, until, if the planet transits the sun, its dark side 
is entirely towards us, and it is a round black spot. 1 

52. Transits of Mercury. — Every few years Mercury 
passes directly across the sun's disk. The transits are 
important in astronomical calculations, but are of little 
interest to observers. So small is the planet that the 
transit cannot be seen with the naked eye, but may be 
seen with a small telescope or a spy-glass. It is simply 
a small black spot on the face of the sun, crossing it in 
a few hours. The next transits of Mercury will occur 
on the following dates : 

November 10, 1894. 
November 4, 1901. 

53. How to observe Mercury. — Mercury is always near 
the sun, and on that account is seldom seen except by 
astronomers. It can only be seen with the naked eye 
about the time of greatest elongation. It may then 
set one and one-half hours after the sun, or rise one 

1 It is important that these phases and their cause should be clearly 
understood. If a careful reading of the explanation does not clear 
the matter up, a diagram, or a representation of Mercury's motion 
by moving any object around an imagined sun, and between that and 
an imagined earth, together with a little study, will make it clear. 
In which direction from Fig. 16 is the earth supposed to be ? 



THE INFERIOR PLANETS. 69 

and one-half hours before it, but the twilight and its 
nearness to the horizon interfere very much with its 
observation. The times of greatest elongation may 
be found in our common almanacs. For a week 
or more before and after these days the planet 
may be looked for. 1 If at these times a strange 
star be seen near the place on the horizon where the 
sun went down, one may be pretty certain that he has 
found the planet. It will not appear very bright, but 
will be as bright as any fixed star would appear in its 
position. The beginner must not be disappointed if 
he have difficulty in finding this planet, or even if he 
fail to find it. The great Copernicus never succeeded 
in finding it ; but this was largely due to his northern 
latitude, where twilight lasts longer. To the naked 
eye the planet looks just like a star; w T ith a small tele- 
scope the phases can be seen, w T hich are its only inter- 
esting features. "When in its most favorable position 
for observation it is always about half full. 

1 In order that astronomical observation may be most successful, 
the body should be observed when at a considerable distance above 
the horizon ; for any one may notice that even upon a very clear night 
stars close to the horizon cannot be seen well, if at all. This direc- 
tion cannot, however, often be observed with Mercury. In general, 
moonless nights are the best for astronomical observation, although 
in the case of the bright planets the moon will interfere but little 
if it is not in their immediate neighborhood. Then, of course, 
the atmosphere and sky must be clear. The atmosphere, however, is 
occasionally quite deceptive. It will sometimes be very unfit for tele- 
scopic work, especially if high powers (Art. 247) be used on the tele- 
scope, when it seems to be perfectly clear ; and nights which seem 
to be hazy may be found to be excellent for observation. The only 
way to determine the matter will be to bring out the telescope and 
try it. 



70 ASTRONOMY. 



VENUS. ? 

Distance from Sun, 67,000,000 Miles. Diameter, 7600 Miles. 
Length of Year, 1\ Months. Length of Day, Uncertain. 
Specific Gravity, 4|. 

54. Relations to the Solar System, and Description. — Be- 
tween the orbits of Mercury and the earth is Venus. 1 
She comes nearer to us than any of the other planets, 
being sometimes only about 25,000,000 miles away; 2 
but she gets twice as much light and heat as the earth. 3 
Venus is almost the same size as the earth, her diame- 
ter being only three hundred miles less than the earth's. 
Notwithstanding her nearness, this planet is very dif- 
ficult to observe. Some astronomers have announced 
markings on its surface and a blunting of its horns 
when crescent-shaped, and have thus deduced a period 
of rotation of about 23J hours; others with equally 
good opportunities have strenuously denied the exist- 
ence of any such markings. Hence, while it is prob- 
able that they exist, they are very obscure and hard 
to see; it is not unlikely that, like Mercury, it ro- 
tates on its axis in the same time it makes a revo- 
lution around the sun. There is strong evidence of 
a dense atmosphere, and it would seem that this 
and its probable thick clouds reflect the sun's light 
so brightly that we never see the surface of the 
planet at all. 

1 Venus, the goddess of beauty and love. Her sign is 9, a mirror. 

2 How is this found ? What is her greatest distance from the earth ? 

3 The amount of light and heat that a planet receives from the sun 
depends upon the square of its distance. The earth is about 1 J times 
as far from the sun as Venus. The square, of 1 J is 2J, or about 2 : 
therefore the earth receives one-half as much heat and light. 



THE INFERIOR PLANETS. 71 

55. Motions and Phases. — As Venus, is also an infe- 
rior planet, she swings from one side of the sun to 
the other, and passes through her phases just like 
Mercury, but on a larger scale. At her greatest 
elongation Venus is forty-seven degrees from the sun, 
and, owing to the great difference in her distances from 
us, her size varies much more than Mercury's. 

Fig. 17 shows the appearance and comparative sizes 




Fig. 17 —The Appeabance and Comparative Sizes of Venus in its Different Phases, 

of Venus when nearly between the earth and the sun, 
when at greatest elongation, and when on the opposite 
side of the sun. 1 

56. Transits of Venus. — These are much rarer phe- 
nomena than transits of Mercury, and have been con- 
sidered to be of great importance, because they have 
hitherto been thought to afford the best opportunity 
of finding the distance from the sun to the earth. To 
find this, stations are chosen on opposite sides of the 

1 The shaded parts of the figure are only intended to fill out the 
disks. The dark part of the moon can sometimes be seen, but not so 
mth Yenus. 



72 ASTRONOMY. 



earth, in the northern and southern hemispheres, as at 
1ST and S in Fig. 18. To the observer at N, Venus 
seems to cross the sun on the line HF ; to the one at 
S, it crosses higher up, on CD. Each observer deter- 
mines carefully where the planet seems to cross, and 




Fig. 18.— Transit of Venus. 

this gives the angular distance between A and B. 
This, with the distance from 1ST to S, determined on 
the earth, and the comparative distances of the earth and 
Venus from the sun, which are found by Kepler's 
third law (Art. 29), w 7 ill enable a person who has a 
knowledge of geometry and trigonometry to find the 
distance to the sun. 1 The real calculation of the dis- 

1 NYS and AYB maybe taken to be similar isosceles triangles.* 

therefore 

NY : VB : : NS : AB. 

But, by Kepler's third law, 

NY : YB : : 1 : 2.61, 

and therefore 

1 : 2.61 : : NS : AB ; or, AB = 2.61NS. 

Suppose that NS is the diameter of the earth, 7918 miles, and that 
AB has been found to be 46". Then AB = 20,666 miles = 46", 
and 1" = 449 miles, which shows that 449 miles, seen at the distance 
between the earth and the sun, subtends an angle of 1". The earth's 
radius, then, if seen from the sun, would subtend an angle of |^ 9 
seconds, or about 8.81", which is the sun's parallax (Art. 33). 
Knowing the parallax and the earth's radius, the solution of a right- 
angled triangle (see foot-note on page 45) gives the distance in miles 
from the earth to the sun. 



THE INFERIOR PLANETS. 73 

tance of the sun by this method is a very complicated 
problem. 1 The observations made during the transit of 
1769 were not completely worked up for fifty years, and 
those made in 1874 and 1882 required years for comple- 
tion. When the distance of the earth from the sun is 
found, the distances of all the other planets are easily 
found by Kepler's third law, and their diameters can 
then be found just as the sun's was found in Art. 34. 

57. The Duration of the Transit determined. — The best 
method of finding the angular distance between the two 
paths across the sun (Fig. 19) is to measure at each sta- 
tion the exact time that it takes Venus to cross the sun. 
This is done by noting down the time when the planet 
first touches the edge of the sun, the first external con- 
tact (A in Fig. 19), and also just when, it breaks away 
from the inside edge 

of the sun, the first ^^ ^^ 

internal contact (B). /f "\ 

From these the time - — -^^@ — w^^ 

when the centre of [ 

the planet crosses the \ / 

edge can be found. \ / 

This is the beginning \^^ ^y^ 

of the transit. In the fig. 19.— the path of Venus across the sun. 

same way, from the 

second internal and external contacts, the time when 

the transit ends is found. And since we know the 

angular distance that Venus passes over in an hour, 

multiplying this by the number of hours occupied in 

1 The simple conditions here supposed arc never realized. Instead 
of two stations, there are many, and no two of them are actually at 
the extremities of a diameter. The earth does not stand still during the 
transit, but rotates on its axis and moves in its path around the sun. 

7 



74 ASTRONOMY. 



the transit will give the lengths, and therefore the po- 
sitions, of the two paths. From these their distance 
apart (AB in Fig. 19) is easily found. 

In 1874 and 1882 the paths were also determined by 
taking during the eclipse a numbef of photographs of 
the sun with Venus upon it. Upon these photographs 
themselves the paths were carefully measured. 

58. The Black Drop. — At the two transits of the last 
century the observers were greatly perplexed at finding, 
when the moment of internal contact came and the 
planet should have separated itself from the inner 
edge of the sun, that it did not do so, but seemed to 
be attached to the edge of the sun for several seconds 
by a sort of neck. This ligament apparently con- 
necting the planet with the edge of the sun is called 
the black drop. This made it very difficult to determine 
the exact time of internal contact. The black drop has 
been found to have been due mainly to the unsteadiness 
of the atmosphere and the imperfections of the tele- 
scopes then used. 

59. The Early Transits. — The first transit of Venus* 
ever observed occurred in 1639. Jeremiah Horrox, a 
clergyman of the Church of England, and only eighteen 
years of age, calculated from the motions of the earth 
and Venus that there would be a transit of the planet 
on a certain Sunday in November. He arranged his 
telescope so as to throw the sun's image upon a screen, 
as explained in the directions given for observing sun- 
spots (Art. 41). During the middle of the day he had 
to be at church, but, when he came back in the after- 
noon, to his great joy hafound the image of the planet 
upon the screen. The next transit occurred in 1761. 
Astronomers now knew the importance of the event, 



THE INFERIOR PLANETS. 75 

and preparations were made to observe it in various 
parts of the world ; but the observations were not satis- 
factory. The next one carhe eight years later, in 1769. 
Astronomers were scattered over all those parts of the 
world from which it could be seen. The observations 
were thought to be satisfactory, and gave a distance of 
95,000,000 of miles from the sun to the earth. This 
was universally accepted for many years, and may still 
be found in older text-books. 

In connection with this transit occurred an incident 
which well illustrates the devotion of scientific men to 
their work. A French astronomer, Le Gentil, had 
been sent out to India eight years before to observe 
the previous transit there. Owing to the war between 
France and England, he was not allowed to land in 
British India. He saw the transit on shipboard, but 
the unsteadiness of the ship prevented him from 
making any valuable observations. As he was there, 
he determined to wait eight years and observe the next 
one. He supported himself by business during these 
years, and made many scientific observations as well. 
" The long-looked-for morning of June 4, 1769, found 
him thoroughly prepared to make the observations for 
which he had waited eight long years. The sun shone 
out in a cloudless sky, as it had shone for a number of 
days previously. But just as it was time for the tran- 
sit to begin, a sudden storm arose, and the sky became 
covered with clouds. When they cleared away, the 
transit was over. It was two weeks before the ill-fated 
astronomer could hold the pen which was to tell his 
friends in Paris the story of his disappointment." 1 

1 NewcomVs Popular Astronomy. 



76 ASTRONOMY. 



Another transit would not occur for over a hundred 
years. 

A part of this transit was visible in the Eastern 
United States, and, under the management of Ritten- 
house, 1 w r as observed at Norristown, Pa., Philadelphia, 
and Cape Henlopen. Although neglected at the time 
by the European astronomers, these observations were 
the most accurate that were made. 

60. The Transit of 1874.— Before the next transit 
came, in 1874, astronomers were certain, from other 
methods of measurement, that a mistake had been 
made in 1769, and that the sun was not so far oft' as 
95,000,000 of miles. This transit was expected to 
settle the matter, and very extensive preparations were 
made to observe it. The transit w r as to occur while it 
was night in the United States and over great part 
of Europe, so Asia and the South Pacific Ocean were 
the best places for seeing it. All of the foremost 
nations of the world sent out expeditions to observe it, 
at an expense altogether of $1,000,000. Our own gov- 
ernment gave $150,000, and sent out eight different 
parties of observers. 

61. The Transit of 1882.— The last transit of Yenus 
was observed on December 6, 1882. It was visible 
over nearly the whole American continent, and the first 
of it over the eastern part of the Eastern continent as 
well. In the transit of 1874, the European observers 
had had such poor results from photographing the sun 
with Venus on it that they determined not to try this 



1 David Kittenhouse, 1732-1796. A farmer's boy who made himself 
a great mathematician and astronomer. He used to calculate eclipses 
on his plough-handles. His life is interesting and instructive. 



THE INFERIOR PLANETS. 77 

method again. The Americans, however, using a dif- 
ferent process of obtaining their photographs, were 
more successful, and, though the results of their efforts 
in 1874 were not all that was expected, it is believed 
that the most valuable outcome of all the observations 
will be found in the American photographs. 

These considerations induced them to try the same 
method in 1882. A series of photographs was taken 
during the transit, about two hundred at each station. 
These photographs are about four inches in diameter, 
and show the sun with Venus on its face as a round 
black spot. They are taken to Washington and meas- 
ured at leisure. 

Eight parties were sent out by the United States 
government, four to the northern and four to the south- 
ern hemisphere. The day was, in general, favorable, 
and good success attended the efforts of the observers. 
Everything possible to be gained by a transit of Venus 
will probably result from this one, and it is no great 
cause of regret that another will not occur until the 
year 2004. 

The results of these two transits are somewhat dis- 
cordant; and astronomers are coming to the conclu- 
sion that even with the best of instruments, in the hands 
of the most experienced observers, observations of the 
transit of Venus do not give us the best methods of 
finding the distance to the sun, but that a method to 
be explained farther on is more reliable. 

62. How to observe Venus. — When at her best, Venus 
is the brightest of all the planets, and much brighter 
than any of the fixed stars. She is never in the 

7* 



78 ASTRONOMY. 



part of the sky opposite to the sun, and at night is only 
to be seen for a few hours after sunset or before sun- 
rise. Tor about nine and one-half months she is seen 
after sunset, and is an evening star ; for the next nine 
and one-half months she is seen before sunrise, and is 
a morning star. When she is to be seen in the evening, 
and when in the morning, are given in all almanacs. 
These facts will generally make it easy to recognize 
Venus. She is not brightest at greatest elongation, but 
when a little nearer the earth than that. At this time 
Venus casts a shadow on a moonless night, and can 
be seen in the daytime with the naked eye, if one 
knows just where in the sky to look for her. The 
smallest telescope will show the phases of Venus, which 
are her most interesting features. Venus's rapid mo- 
tions among the stars, both direct, that is, towards the 
east, and retrograde, towards the west, should be noted. 
(Art. 24). 



THE EARTH. 79 



CHAPTER IV. 

THE EARTH. 

Distance from the Sun, 93,000,000 Miles. Average Diameter, 
7918 Miles. Length of Year, 365} Days. Length of Day, 
24 Hours. Specific Gravity, 5 J. One Satellite. 

63. The Earth's Shape.— The earth is the planet 
which conies next to Venus, and, like the rest of the 
planets, is round, or, more properly, spherical. The 
facts which led the ancients to believe that the earth 
was round have already been given (Art. 7), but other 
proofs of this fact are now known. One of the best 
known of these is that men have frequently travelled 
around it in almost every possible direction. This 
proves that it is rounded, but not that it is certainly a 
sphere. 1 

Another proof commonly given is that, when we 
watch a ship sailing away from the land, we notice 
that its hull is hidden first, then its lower sails, and last 
of all its highest sails. The water takes just the shape 
of the surface of the earth ; and, since this gradual dis- 
appearance is noticed upon the water everywhere, and 
ships disappear just as fast in one direction as in an- 
other, the surface of the water at least must be round. 2 

1 Why not ? 

2 The difference of level on the surface of still water is 8 inches in 
1 mile. In 2 miles it is not twice 8 inches, but the square of two 
multiplied by 8 inches, 4 X 8, or 32 inches. In 3 miles it is 9 X 8, 
or 72 inches, and so on. If your eye were at the surface of the water, 



80 ASTRONOMY. 



A still stronger proof is given by the eclipses of 
the moon. As will be explained hereafter, the moon 
is eclipsed by getting into the earth's shadow. "When 
the moon is passing into the shadow or coming out of 
it, the edge of the shadow as seen upon the moon is 
always round. Many hundreds of eclipses of the moon 
have been seen, and at these different times every side 
of the earth has been turned towards the moon. The 
earth must look round, then, from every side, and must 
be a sphere. 1 Another convincing proof of the spheri- 
cal shape of the earth will be given when the method 
of finding the size of the earth is explained (Art. 65). 

Notwithstanding the numerous and unanswerable 
proofs of the roundness of the earth, there still seem 
to be a few people who deny it. A few years since, 
Mr. John Hampden, of England, wrote a book to 
prove that the earth is flat. He afterwards offered to 
bet five hundred pounds (twenty-five hundred dollars) 
that he was right. The bet was taken, and to settle 
the question a part of an English canal, where there 
were two bridges six miles apart, was chosen. Half- 
way between the bridges a rod was put up. When a 
telescope was set up at one end of the six miles and 
pointed towards the bridge at the other end, the place 
on the rod which was just as far above the surface of 
the water as the bridges, was found to project several 
feet above the line of sight. The referee in the case 
decided that this proved the rotundity of the earth : so 
Mr. Hampden lost his money. 

how tall a ship's mast would be hidden by 10 miles of water ? How 
far out at sea could a mountain 3 miles high be seen ? (154+ miles.) 
1 What different shapes might the earth have and yet look round 
from some sides ? 



THE EARTH. 81 



64. Apparent Deviations from the Roundness of the 
Earth. — One can hardly believe at first that the earth is 
round, when he thinks of the hills and mountains scat- 
tered so thickly over its surface. But, when compared 
with the great earth, these irregularities are insignifi- 
cant. If on the surface of a globe one foot in diameter 
an elevation were constructed proportionate in size to 
the highest mountain on the earth, it would be less 
than j^-q of an inch high, and could not be seen at all 
one foot away. If the loftiest of the Himalayas or of 
the Andes is so trifling, we can see how insignificant 
the hills and even the mountains about us must be. If 
an exact model of the earth one foot in diameter were 
made, a foot away it would seem to be perfectly smooth 
and round. "What appears to us to be the great rough- 
ness of its surface does not, then, at all destroy the 
roundness of the earth. 

65. Size of the Earth, how determined. — Since the earth 
is a sphere, its circumference is a circle, and, like every 
other circle, contains three hundred and sixty degrees. 
So, if we can find the length of one of these degrees, 
multiplying that length by 360 will give the circumfer- 
ence of the earth. 

In 1764-65, Mason and Dixon (who came over from 
England to mark the boundary between Pennsylvania 
and Maryland, still called Mason and Dixon's line), 
at a point in the southeastern part of Pennsylvania, 
carefully observed the height of a certain star above 
the northern horizon, then measured a straight line 
directly south until the star at the same time of 
day (why ?) was one degree nearer the horizon than 
when they started (Art. 20). Then they knew that 
they had measured just one degree of the earth's cir- 



82 ASTRONOMY. 



cumference. 1 Parts of the earth's circumference have 
been measured with the utmost exactness many times 
in different parts of the earth. In other places the 
whole length of the line running north and south has 
not been measured upon the ground, as Mason and 
Dixon measured theirs, but the corners of a row of 
triangles extending from one end of the line to the 
other have been marked. Then, by measuring one 
side of the first triangle and the angles of all the tri- 
angles, the whole distance is calculated by trigonom- 
etry. This is a much more accurate way than to try 
to measure the whole distance, on account of the hills 
and mountains that would interfere with direct meas- 
urement. Very long lines have been measured in 
this way, and the number of degrees from one end to 
the other found by observing the stars. The average 
length of one degree of the earth's circumference has 
been found to be about 69 miles, and the whole cir- 
cumference, then, is a little less than 25,000 miles. 
These measurements of the size of the earth also prove 
that it is a sphere. 

66. The Earth Flattened slightly at the Poles. — In meas- 
uring these lines, it was found that a degree of a me- 
ridian near one of the poles of the earth is a little 
longer than a degree near the equator. This shows 
that the degree near the pole is part of a greater circle 
than the degree near the equator, 2 or that the earth is 

1 Students who understand geometry should work out the proof of 
this. 

2 This may he hard to see ; if so, let the student draw a circle flat- 
tened above and below. He will see that where the equator bulges 
out there is a sharper curve than at the flattened parts. If the curva- 
ture of the upper or lower part be carried on around till it meets, 



THE EARTH. 83 



slightly flattened at the poles. The circumference of 
the earth which passes through the poles is not, then, a 
perfect circle, but is an ellipse ; but it is so nearly a circle 
that, if it were accurately drawn, no one could dis- 
tinguish it from a circle. The distance through the 
earth from north to south pole is twenty-six miles 
less than the diameter from one side of the equator to 
the other. This is much greater than the height of any 
mountain, and, as we shall learn hereafter, the bulging 
out of the earth's equator produces some important 
astronomical effects, but it would make no appreciable 
difference in the shape of a globe. If, then, an exact 
model of the earth of a moderate size were made, the 
sharpest eye could not see but that it was exactly round, 
a perfect sphere. From the circumference the earth's 
diameter is easily found by arithmetic. Its average 
length is 7918 miles. 

67. Weight and Density of the Earth. — Many attempts 
have been made to determine the weight of the earth. 
The first was made in this way. As we all know, a 
plumb-line points directly down to the centre of the 
earth. But if the plumb-line be held near a mountain, 
the attraction of the mountain pulls it a little to one 
side. If the mountain were not there, the plumb-line 
would point to a certain place among the stars ; but the 
attraction of the mountain makes it point to a differ- 
ent place. The difference between these two places 
in the sky shows how far the mountain pulls the plumb- 
line aside. The size of the mountain is then meas- 
ured, and the average weight of the rocks that make 



or completes a circle, this circle will run outside of the middle parts 
of the flattened one. 



84 ASTRONOMY. 



up the mountain is found ; these Avill give the weight 
of the mountain. Knowing this, and how much the 
mountain pulled the plumb-line from the earth's cen- 
tre towards itself, the weight of the earth is calcu- 
lated. Although this was the first method employed, 
it is not altogether reliable, because we cannot be cer- 
tain that the weight of the mountain has been correctly 
found. 

The best method of finding the mass of the earth is to 
measure the force with which a large lead ball attracts a 
small body to itself. The weight of the small body shows 
how much the earth attracts it, for the weight of any body 
is caused wholly by the earth's attraction. Then if the 
attracting force of the lead ball, the attracting force of 
the earth, and the weight of the lead ball be all known, 
the mass of the earth may be found. 1 This experiment has 
been repeated many hundreds ot times with the greatest 
care, and as the result the mass of the earth is represent- 
ed by a weight of about 6,000,000,000,000,000,000,000 
tons. This shows that the specific gravity of the earth 
is about 5 \ ; that is, it is five and one-half times as 
heavy as a globe of water of the same size. The rocks 
and soil at the surface of the earth are not nearly so 
heavy as this, being generally only two or three times 
as heavy as water. The inside of the earth must, then, 



1 To solve this the distances of the body from the centre of the earth 
and from the ball ought to be known. Calling these T> and d, and 
denoting by M and m the masses of the earth and ball respectively 
we have, from natural philosophy, — 

7YI IVt 

Attracting force of ball : attracting: force of earth ::_:__. 
3 & d 2 D 2 

In this proportion everything is known but M, which may there- 
fore be easily found. 



THE EARTH. 85 



be very much heavier than its surface. This is clue to 
the condensation caused by gravitation. 

68. The Mirth's Rotation on its Axis. — The ancients 
generally believed that the earth stood still, and that the 
sun and stars revolved about it every day from east to 
west. But it is now known that these motions of the 
heavenly bodies are only apparent, and are caused by 
the rotation of the earth upon its axis from west to east 
once a day. One proof that the earth turns thus upon 
its axis is that all of the thousands of stars do thus seem 
to revolve about the earth in exactly the same time.. 
The distances of the stars vary greatly, but all are at 
enormous distances from us, so that it is impossible to 
suppose that they all revolve about such enormous cir- 
cles in so short a time, and in exactly the same time. 
The only possible explanation of their apparent motion 
is that the earth turns on its axis in the other direc- 
tion. 

Again, if the earth thus rotates upon its axis, the top 
of a tower must move through a larger circle in the 
same time, and hence move faster, than the foot of the 
tower. If a stone were dropped from the top of the 
tower upon the eastern 1 side, in falling through the air 
it would still keep the forward motion of the top of the 
tower. And, since the top moves faster than the bot- 
tom, the stone while falling would move faster east- 
ward than the bottom, and would strike the ground a 
little way east of the foot of the tower. If the tower 
were at the equator, and 500 feet high, the stone would 
fall about two inches from its foot. This experiment 
has been tried many times from towers, and in the 

1 Why not upon the western side ? 
8 



86 ASTRONOMY. 



shafts of deep mines, and from it we have another proof 
of the rotation of the earth. 1 

Until recently astronomers thought that the time of 
the rotation of the earth remained exactly the same 
from century to century, and that there was therefore 
no change in the length of the day. But it is now 
thought that the ocean tides, which move around the 
earth in the other direction, by their friction may be 
gradually making the rotation of the earth slower, and 
thus slowly lengthening the day. But as a day is, at the 
most, only -^ of a second longer than it was 2500 years 
ago, we may consider the length of the day as practi- 
cally invariable. 

69. Revolution of the Earth about the Sun. — As has 
been said, the sun seems to move around the whole 
sky among the stars once a year. There is abundant 
evidence that this, too, is only apparent, and that it 
is the earth that really moves about the sun in this 
time. 

70. The Shape of the Earth's Orbit — If the apparent 
size of the sun is carefully measured with a telescope 
every day in the year, it will be found to be largest 
about the 1st of January, and to grow smaller every 
day until about the 1st of July, when it will be small- 
est ; then it will daily grow larger until about the 1st 
of January again. As we cannot suppose the sun's 
size to change in this way, we are forced to conclude 
that the sun seems largest at the 1st of January be- 
cause we are then nearest to it, and that as it seems to 

1 The student may have heard of the man who proposed to travel 
from one place to another by going up in a balloon, and after waiting 
until the earth turned around under him, to come down. Why 
would his plan not succeed ? 



THE EARTH. 87 



grow smaller from day to day, we must be going far- 
ther from it every day ; about July 1, when it seems 
smallest, we must be farthest from it, just as a man 
seems smaller the farther off he is. If the earth's dis- 
tance from the sun varies in this way, its path about 
the sun cannot be a circle with the sun in the centre. 
It is an ellipse, like the paths of the other planets. The 
measurements just referred to show T that at the 1st of 
January the sun's diameter appears to be -fa longer than 
at the 1st of July, so that the earth must be about fa 
nearer the sun on the former than on the latter day. 
As the average distance of the earth from the sun is 
93,000,000 miles, we are more than 3,000,000 miles 
nearer the sun at the 1st of January than at the 1st 
of July. 

71. The Eccentricity of the Earth's Orbit — Its Effect 
upon Climate. — Since the difference of greatest and least 
distances is -fa, one must be fa greater and the other 
-fa less than the average distance. This -fa is the eccen- 
tricity of the earth's orbit (Art. 25). It may seem 
strange that we should be nearest the sun in winter 
and farthest from it in summer, but we shall presently 
learn that summer and winter are due to other causes 
than distance from the sun. Although these differ- 
ences of distance are small compared with the whole 
distance, and the sharpest eye could not distinguish 
the earth's orbit from a circle (Art. 25), yet they are 
really considerable, and the earth actually receives fa 
more heat 1 on the 1st of January than on the 1st of 
July. This makes our 2 winters slightly warmer and 



1 For the method of finding this, see foot-note, p. 70. 

2 This refers to the northern hemisphere. 



88 ASTRONOMY. 



our summers slightly cooler than they would other- 
wise be. But, as will be explained presently, after 
some thousands of years this will be reversed, and we 
shall be farthest from the sun in winter, and nearest 
to it in summer. 

72. Precession of the Equinoxes. — The student will re- 
member that the sun in its apparent yearly revolution 
around the sky along the ecliptic crosses the celestial 
equator in two points called the equinoxes (Art. 30). 
If the equator were a visible line in the sky, it would 
seem to be a great circle among the stars, running en- 
tirely around the earth ; half of it only could be seen 
at once. On March 20 the sun would be exactly on 
this line, crossing it from south to north ; the place of 
crossing would be the vernal equinox. Day by day the 
sun would be seen moving to the east anions the stars 
(if they could be seen at the same time as the sun), and 
also getting farther and farther above the line until 
June 21, when it would be 23J° above or north of the 
line. Then, as it moved on in its eastward course, 
it would draw nearer and nearer to the line again, 
crossing it from north to south on September 21, the 
autumnal equinox. For the rest of the year its path 
would be a similar curve south of the equator, coming 
back to cross the equator on March 20 again. But this 
time the sun would cross the equator a little before it came to 
the place where it crossed a year before. The equinox, or 
place where the sun crosses the equator, moves back- 
ward or westward every year. The same w^ould hap- 
pen at the other equinox in September ; there, too, the 
sun would cross the equator before it got to the cross- 
ing-place of the year before. This moving backward 
of the places where the sun crosses the equator is 



THE EARTH. 89 



called the precession 1 of the equinoxes. This motion is 
extremely slow. It would take the vernal equinox 
about 26,000 years to move entirely around the equa- 
tor once in this way. The sun crossing the equator 
only a very little way farther back every spring would 
cross it about 26,000 times before coming to its first 
crossing-place again. 

If the earth were perfectly round there would be no 
such motion of the equinoxes : the sun would always 
cross the equator at the same places. But, as we saw 
in Art. 66, the earth is not quite a perfect sphere. It 
is flattened at the poles, or, which is the same thing, 
there is a bulging or protuberance about the equator. 
It is the attraction of this equatorial protuberance by 
the sun and moon that causes the precession of the 
equinoxes. 

73. Effects of the Precession of the Equinoxes. — As has 
been said in Art. 31, the right ascension 2 of a heavenly 
body is the distance (in degrees) from the vernal equi- 
nox eastward, or forward, to the body. And as the ver- 
nal equinox moves backward, the right ascensions of 
the stars must be growing greater. As the precession 
itself is so slow, this change is also slow, but after 
many years it becomes considerable, and it was by no- 
ticing this increase in the right ascensions of stars that 
Hipparchus (Art. 6) discovered the precession of the 
equinoxes more than two thousand years ago. The 
fact that the first sign of the ecliptic does not now 
coincide with the first constellation of the zodiac (Art. 
32) is now explained. As there stated, when the con- 



1 From Latin prcecedere, to go before. 

2 What corresponds to right ascension upon the earth ? 

8* 



90 ASTRONOMY. 



stellations were named, the vernal equinox was prob- 
ably at the beginning of the first constellation, so that 
the signs of the ecliptic received the names of the con- 
stellation in which they lay. But since that time the 
equinox has moved nearly 30° backwards, and the first 
sign of the ecliptic coincides with the twelfth constel- 
lation. 

Another effect of the precession of the equinoxes is 
to change slowly the direction in which the axis of the 
earth points. The backward motion of the equinoxes 
causes the north pole to move around in a circle once 
in 26,000 years. 1 Now the north end of the earth's 
axis points almost directly to what is called the north 
star, and it will continue to point almost towards it 
for many years to come. But the earth's axis has not 
always pointed towards this star, nor will it always 

1 This motion of the earth's axis and the whole subject of the preces- 
sion of the equinoxes constitute one of the most difficult points in 
astronomy. The following illustration may help to make it clear : 

Take an apple to represent the earth. Call the stem the north pole, 
and mark a line around the middle of the apple for the equator. If 
this apple be floated in a bucket of water so that just half of it is 
above the water, with the stem leaning about 23J° from the per- 
pendicular, the position of the earth is well represented. The sur- 
face of the water is the ecliptic, and where the apple's equator crosses 
the water-line are the equinoxes. If the apple be now twisted around 
so that the stem shall move in a circle, leaning in every direction, but 
always about 23J° from the perpendicular, half of the apple being 
always in the water, this revolution of the north pole is represented. 
And it will be seen that the surface of the water is continually cross- 
ing the apple's equator in new places as the apple turns. This repre- 
sents the precession of the equinoxes. 

This motion has also been compared to the motion of a top when 
it is " dying out." It then leans outward and slowly revolves. One 
such revolution of the top represents the 26,000 year revolution of 
the earth's axis. 



THE EARTH. 



91 



do so in the future. And, as it moves slowly around 
in its journey of 26,000 years, it will point in turn 
to every star that lies in its circular path. So that 
future generations will have to use other stars for 




Fig. 20.— The 26,000 Year Path of the North Pole 
amoxg the stars. 

their north stars. Fig. 20 shows the path of the north 
pole among the stars as caused by the precession of 
the equinoxes. Some of the stars which generations 
in the far future will probably use as north stars, are 
marked. 

74. Nutation. — If the attraction of the sun and moon 
upon the protuberance about the earth's equator were 
always the same, the precession of the equinoxes would 
cause the north pole to revolve in a perfect circle. 
But, on account of its own motions, the moon's at- 
traction is always changing, growing greater and less 
alternately. This causes the pole to revolve in a wavy 
curve, and not in a perfect circle, as represented in 



92 



ASTRONOMY. 



NOON DAY 
FAY 



Fig. 20. The real path of the north pole in the sky is 
a circular, wavy line, crossing and recrossing the circle 
in Fig. 20. But these wavings are so small that they 
cannot be shown in the circle in Fig. 20. Yet they are 
of much importance in astronomy. It is this waving 
backward and forward that is called nutation. 1 

75. The Seasons. — When the sun is nearly overhead 
it gives us much more heat than when it is far down 
in the sky. This is proved every day. In the morn- 
ing or in the evening the sun's rays are very slanting, 
and it is much cooler than at noon, when the sun shines 
much more directly down upon us. 

Fig. 21 represents two rays of equal breadth striking 

the earth at noonday and 
at evening respectively. 
It is plain that the in- 
clined or evening ray has 
to warm a much larger 
surface (BC) than the di- 
rect or noonday ray; and 
each spot cannot, there- 
fore, receive so much 
heat. 

The changing seasons 

-Sun's Rays striking the Earth are due to the Same Cause, 
at Different Inclinations. ,, . , . 

the varying inclinations 
of the sun's rays. Fig. 22 represents the earth with 
its important circles drawn upon it. It will help to 
explain the seasons. On March 20 the sun is directly 
over the equator, exactly in front of the middle of the 
figure. And since the sun lights up just half of the 




Fig. 21, 



1 Nu-ta/tion, from Latin nutatio, nodding. 



THE EARTH, 



93 



earth at one time, its light extends both to the north 
and south poles. If the figure be held up-just in front 
of the eyes, and be supposed to rotate about the axis 
from left to right, or, much better, if a globe be held in 




Fig. 22. — The Earth and its Important Circles. 



this position and made to rotate, it will be seen that, 
since the sun is on the equator, it will rise directly east 
of us at six o'clock in the morning, and set directly 
west of us at six o'clock in the evening. Day and 
night will be equal all over the earth. (See Fig. 23.) 
Hence the name of this time of year, the equinox. This 
is the beginning of spring, so it is the venial equinox. 
As the sun moves on in its yearly journey around the 



94 ASTRONOMY. 



sky, 1 it is of course always directly over the line marked 
ecliptic in the figure. 

Moving from left to right, the sun in three months 
goes one-fourth way around its circular path, and on 




Fig. 23. — Day and Night at the Equinoxes. 

June 21 is to the right of the figure (22), directly beyond 
D. As the sun is now 23J° above the equator, it shines 
around 23 J° beyond the north pole to A; but below 
the equator it shines only to B, 23J ° short of the south 
pole. Right of AB is day, left of AB is night. As 

1 It must not be forgotten that this apparent motion of the sun 
about the earth is really due to the earth's yearly motion about the 
sun, and that this causes the change of seasons. But the explanation 
is simpler if the sun be supposed to revolve about the earth. In the 
same way we commonly say, " The sun rises," or " The sun sets," 
although these things are really due to the rotation of the earth. 



THE EARTH. 



95 



the earth rotates while the sun is here, the sun rises in 
the northeast and sets in the northwest, and we in the 
north temperate zone have our longest days and short- 




Fig. 24.— Day and Night at the Summer Solstice. 

est nights. This is the beginning of summer, — the 
summer solstice. (See Fig. 24.) 

For the next three months the sun moves on around 
the earth, now of course back of the figure (22), and on 
September 22 is directly behind the middle of the fig- 
ure and on the equator, making day and night again 
equal everywhere. This is the beginning of autumn, 
— the autumnal equinox. (See Fig. 23.) 

In three months more the sun is directly to the left 
of C, 23i° south of the equator. NW the half of the 
earth to the left of AB is lighted up, while the half to 
the right is in darkness. The sun shines 23J° beyond 
the south pole ? and not at all within 23J° of the north 



96 ASTRONOMY. 



•pole. Now, as the earth rotates, the sun rises in the 
southeast and sets in the southwest; our days are 
shortest and our nights longest. It is the winter solstice, 
— the beginning of winter. Another three months, and 
the sun comes to the vernal equinox again. 

76. The Seasons in the Southern Hemisphere. — South 
of the equator the seasons are exactly opposite to ours. 
When the sun is nearest over our heads, and shines 
most directly down upon us, in the southern hemi- 
sphere it is lowest down in the sky, and its rays are 
most slanting. There the winter months are June, 
July, and August; the summer months, December, 
January, and February. 

77. Why the Days and Nights are Unequal. — Figs. 22 
and 24 show why, except at the equator, day and night 
are generally unequal. When the sun is to the right 
of D (Fig. 22), it shines upon more than half of each of 
the northern parallels of latitude (the tropic of Cancer, 
for instance) ; and therefore every place north of the 
equator, since it revolves every day through a circle of 
latitude, has sunlight for more than half of the twenty- 
four hours. It is evident, too, that the farther north 
one goes, the greater is the part of the circle in the sun- 
light, and therefore in summer the days grow longer 
and the nights shorter as we go north. 1 At the pole 
itself the sun shines for half the year, for from the 
vernal to the autumnal equinox the sun is north of the 
equator, and therefore less than 90° from the north 
pole, so that it must always shine upon the pole. At 
other places in the frigid zone the sun shines day 

1 In Edinburgh, Scotland, on June 21, the sun rises about half-past 
three in the morning, and does not set until half-past eight in the 
evening, while twilight lasts all night. 



THE EARTH. 97 



after day without setting, the time being greater as the 
place is nearer to the pole. South of the equator the 
nights are longer than the days, and the south pole is 
having a six-months' night. Six months later the con- 
ditions are reversed. The people of the southern hemi- 
sphere have the long days and we the long nights. 

78. Duration of the Seasons. — Astronomically, the sea- 
sons begin and end with the equinoxes and solstices. 
Spring begins March 20, and ends June 21 ;* summer 
begins June 21, and ends September 22; and so on 
through the year. Our almanacs agree with this divi- 
sion of the year ; but popularly, in the United States, 
spring begins March 1, and summer June l. 2 

If the number of days from the vernal to the au- 
tumnal equinox be counted, it is found to be seven 
more than the number from the autumnal to the vernal 
equinox. And if the time be counted more exactly, it 
is found that the sun is north of the equator about eight 
days longer than he is south of it. This is clue to the 
fact that the sun is nearer to one end of the earth's 
orbit than to the other ; and the earth moves through 
the larger end of its orbit in our spring and summer, 
taking a longer time to do it. In the southern hemi- 
sphere this is reversed. There spring and summer are 
eight days shorter than autumn and winter. This 
seems to make the temperature of the northern hemi- 
sphere milder than that of the southern. In time the 
precession of the equinoxes will reverse these conditions. 

79. Causes of Heat and Cold at Various Seasons. — As 

1 Remember that these may vary a day (see note on p. 40). 

2 In England, February, March, and April are commonly called 
the spring months, May, June, and July the summer months, and so 
on through the year. 





98 ASTRONOMY. 



has been shown, the sun shines most directly down 
upon us in summer, and most obliquely in winter. This 
would make our summers warmer than our winters. 
Besides, when the sun shines most obliquely upon us, 
the rays of heat pass through a greater thickness of 
air, which absorbs more of their heat, leaving less to 
reach the earth. 1 A third reason is that in summer 
the days are longer than in winter. 

80. Why the Greatest Heat and Cold occur after the Sol- 
stices. — On June 21 the sun shines most directly down 
upon us, and also for the longest time ; it may seem 
strange, then, that we have our hottest weather not 
at that time, but several weeks later. It is true that 
we are getting the most heat from the sun on June 21, 
but for several weeks after that day we receive more 
heat than we lose by radiation, and the weather grows 
hotter; just as a man who earns even a little more 
than he spends is constantly growing richer. About 
June 21 our savings of heat are the largest, for we then 
get most, but we are still saving some, though less and 
less every day, for several weeks, and are thus growing 
richer in heat until the latter part of July or the begin- 
ning of August. In the same way our coldest weather 
generally comes not at the winter solstice, but perhaps 
a month later. On December 21 we receive the least 
heat from the sun, but for some weeks afterwards we 
continue to lose more than we get, and are growing 
colder. For the same reason the hottest part of the 
day is not at noon, but some time in the afternoon. 

81. Geographical Zones. — Fig. 22 also shows the well- 
known geographical zones. The torrid zone extends 



1 Construct a figure showing this. 



THE EARTH. 99 



23J° on each side of the equator. Its boundaries are 
the lines where the sun turns back at the solstices. Its 
upper boundary is called the tropic of Cancer ; the sun 
is directly over this line on the 21st of June. It then 
enters the fourth sign of the ecliptic, Cancer (Art. 30), 
from which the tropic is named ; the sun now begins 
to go back towards the equator. Six months later it 
has gone entirely across the torrid zone to the tropic 
of Capricorn, which takes its name from that of the 
tenth sign of the ecliptic, which the sun now enters 
and begins its journey back towards the equator again. 
The torrid zone, then, is the part of the earth's sur- 
face which the sun some time in the year shines directly 
down upon. It is the hottest part of the earth's sur- 
face. In Art. 5, when the sun was supposed to be to 
the right of D, in Fig. 22, it was seen that the sunshine 
would extend 23J° beyond the north pole to A. As 
the earth rotates, the sun constantly shines over all the 
area within 23 J° of the north pole. The circle which 
is everywhere 23 J° from the north pole is the boundary 
line between the north frigid and the north temper- 
ate zones. It is the Arctic circle. At all points of 
this circle the sun would just escape setting on June 
21. At the same time the sun does not shine within 
23J° degrees of the south pole. This is the south 
frigid zone, and at the Antarctic circle on the 21st 
of June the sun does not rise. 

Six months later the sun is at the winter solstice : 
now the sun does not rise in the north frigid zone, 
and does not set in the south frigid zone. Here, 
when they shine at all, the sun's rays are always very 
oblique, and these are the coldest portions of the earth's 
surface. 



100 ASTRONOMY. 



The temperate zones lie between the frigid and 
torrid zones. Here the sun is never directly over- 
head, yet it rises and sets every day during the year. 
Their name fitly describes the temperature of these 
zones. 

82. Effects of a Change in the Angle between the Equator 
and the Ecliptic. — If the angle between the equator and 
the ecliptic should increase (see Fig. 22), the torrid and 
frigid zones would widen, while the temperate zones 
would grow narrower. The sun would rise higher in 
the sky in summer, and sink lower in winter. This 
would make our summers hotter and our winters colder. 
If the angle should decrease, the torrid and frigid 
zones would decrease also, but the temperate zones 
would widen. 1 The sun would not rise so high in the 
sky in summer or sink so low in winter. Our sum- 
mers would be cooler, our winters warmer. If there 
were no angle between the two, the sun would always 
be directly over the equator, day and night would be 
equal everywhere through the whole year, and there 
would be no change of seasons. The equator of the 
planet Jupiter makes a very small angle with its orbit, 
so that there can be scarcely any change of seasons 
upon it, while on Mars the angle is somewhat greater 
than upon the earth, and there the changes are greater 
than here. This angle between the equator and the 
ecliptic is called the Obliquity of the Ecliptic. 

As a matter of fact, this angle does change slightly, 
but the change is very slow, and will never make much 
difference in the size of the angle, so that from this cause 

1 How many degrees wide would the different zones be if the angle 
between the equator and the ecliptic were 15° ? 30°? 49° ? 45°? 






THE EARTH. 101 



there will never be any considerable change in our 
seasons. 1 

83. Measures of Time. — The three most natural divis- 
ions of time are the year, which is the time the earth 
takes to revolve about the sun ; the month, which is 
based upon the time the moon takes to revolve about 
the earth : and the day, which is the time of the ro- 
tation of the earth upon its axis. AVe see, thus, that all 
our measures of time depend upon astronomy. The 
finding and keeping of correct time all over the earth 
is always done by astronomy, and is one of its most 
valuable uses. 

84. The Sidereal Day. — From the time a star crosses 
our meridian 2 until it crosses it again is called a side- 
real 3 day. This is the exact time in which the earth 
turns on its axis. As has been said, this time is prac- 
tically invariable. It may be well to recall the fact 
that the word "day" is used in two senses. As op- 
posed to night, it means the period of daylight, about 
twelve hours ; but as used here and upon several pages 
following this, it includes both daylight and darkness, 
twenty-four hours. 

85. The Apparent Solar Day. — If the sun, like a star, 

1 The angle between the equator and the ecliptic is now decreasing 
about 45" every hundred years. This will continue for many cen- 
turies ; then it will grow greater again, and so vibrate backward and 
forward. The angle will never be as much as one and one-half de- 
grees greater or less than at present, and will be thousands of years 
in making one such vibration. 

2 A star is said to be on our meridian when it is directly over the 
meridian of the earth passing through the place where we are. If 
the star's declination (Art. 31) is equal to our latitude, it is then ex- 
actly overhead ; if not, it is directly north or south of us. 

3 Side'real, from Latin sidus, a star. 

9* 



102 ASTRONOMY. 



were always at the same place in the sky, a solar day 
would be just the same as a sidereal day. But we have 
learned that the sun moves entirely around the heavens 
every year; that is, it moves through 360° in 365 
days, or about 1° every day, towards the east; this 
distance is about twice the sun's diameter, for the sun's 
diameter is about one-half of a degree. Now, the earth 
rotates on its axis in the same direction, and when its 
rotation has brought us around under the sun's place 
of the day before, the sun has moved one degree farther 
east, and the earth must turn that much farther to 
bring us under the sun again. Thus the time from 
noon to noon by the sun, which is the apparent solar 
day, is about four minutes longer than the sidereal 
day. 1 

86. The Apparent Solar Days not Equal in Length. — 
It has just been shown that the solar day is longer than 
the sidereal day, because the earth has to turn a little 
farther than a complete rotation to catch the sun. But 
these forward movements of the sun are not the same 
every day, so that the earth does not turn the same dis- 
tance every day to catch the sun, and the solar days are 
therefore unequal. There are two reasons why the sun 
does not move the same distance forward every day. 

87. First Cause of Inequality in Solar Days. — As shown 
in Art. 29, when the earth is in perihelion (Art. 25) it 



1 As Proctor points out, this fact bears upon a curious error often 
found in our geographies and other text-books. It is commonly said 
that while the earth revolves about the sun once it rotates upon its 
axis 365J times. In fact, the earth rotates 366£ times during the 
year, although there are but 365J da} y s in the year. For the time of 
rotation is four minutes less than a day, as explained above, and there- 
fore there is one more rotation than there are days. 



THE EARTH. 103 



moves more rapidly than in any other part of its orbit, 
while at aphelion it moves most slowly. Since the sun's 
apparent motion among the stars is really the earth's 
motion, about January 1, when the earth is at peri- 
helion, the sun will move farther every clay than usual. 
This will be further increased by the fact that because 
the sun is then nearest to us, it w T ill seem to move still 
more rapidly. About the 1st of January, then, the 
solar days are longer than the average. And since 
about the 1st of July the sun really moves more slowly 
than usual, and from its great distance seems to move 
still more slowly, the days then are shorter than usual. 

88. Second Cause of Inequality in Solar Days. — The 
ecliptic in which the sun moves is inclined to the equa- 
tor, and when the sun is near the equinoxes its motion 
of a degree a day is on the hypothenuse of a right-angled 
triangle, but our eastern motion to overtake him is 
parallel to the equator, along the base of the triangle, 
and not so great. This would make the days about 
the equinoxes shorter than the average. 

At the solstices the sun moves nearly in the tropics 
of Cancer and Capricorn, parallel to the equator. And 
as the sun moves through his regular daily distance 
along these small circles, it moves more than a degree 
along them each day. This makes the days about the 
solstices longer than the average. 

89. Mean Solar Day. — On account of these constant 
changes in the length of the apparent solar day, it is 
not a good measure of time. But the average or mean 
of all the solar clays in the year is taken as the standard 
day. This is the day which our clocks and watches 
keep, and which is divided into twenty-four hours, and 
these into minutes and seconds. 



104 ASTRONOMY. 



90. The Equation of Time. — Sun time is got from the 
sun either by a sun-dial, or by setting the clock at 12 
when the sun is exactly on the meridian. From the 
two causes given in Arts. 87 and 88, sun time agrees 
with true or mean time only on four days of the year. 
These are 

April 15, June 15, September 1, December 24. 1 

On the following intervening days, the difference 
between true time and sun time is greatest : 

February 10, sun time 15 minutes slow. 
May 14, " 4 " fast. 

July 25, " 6 " slow. 

November 2, " 16 " fast. 1 

The difference between true time and sun time is 
called the equation of time. Our common almanacs give 
the equation of time for every day of the year in a col- 
umn on the page which gives the calendar of the month. 
In getting the time from a noon-mark or sun-dial, the 
time as thus found must be corrected as indicated in 
the almanac. 2 The times of sunset and sunrise as given 
in the almanac are in mean or true time at the lati- 
tude for which the almanac is calculated. 3 This time 

1 These dates may vary slightly. 

2 If the column in the almanac is headed Sun Slow, the number 
of minutes must be added to sun time. If it is headed Sun Fast, 
they must be subtracted. 

3 According to the almanac, forenoon and afternoon are seldom of 
the same length. The time from sunrise until apparent noon (when 
the sun is on the meridian) is just the same as the time from apparent 
noon until sunset. But mean noon is understood in the almanac. If 
Vhe sun is slow, he rises too late, and the forenoon is shorter than the 
afternoon. If fast, the sun rises early, and the forenoon is the longer. 



THE EARTH. 105 



will be exact only where the sun rises over a level sur- 
face. 

91. How Time is Found. — Time is sometimes got 
from the sun as just mentioned, but it is generally and 
most accurately obtained by observations of the stars at 
astronomical observatories. By using a small telescope 
called a Transit (Art. 249), we can find out just when 
some well-known star crosses the meridian. The time 
when this ought to occur is given in the Nautical Alma- 
nac (seep. 170), and if the clock does not show the same 
time, we know how much too fast or too slow it is. 

In 1883, by agreement of the railroad managers, four 
time-centres were established in the United States, viz., 
the meridians 5 hrs. (75 degrees), 6 hrs., 7 hrs., and 8 
hrs., west of Greenwich. All our railroads, and our 
time-pieces generally, now run upon the time of the 
nearest or most convenient of these meridians. Correct 
time is sent along the railroads every day by telegraph ; 
and in many cities large balls, called time-balls, are let 
fall from high points exactly at noon each day. 

92. Civil and Astronomical Days. — Civil or ordinary 
days begin and end at midnight, and are divided into 
two equal parts, each twelve hours long. The hours 
from midnight to noon are marked a.m., the first letters 
of the Latin words ante meridiem, " before noon." Thus, 
5 a.m. means five o'clock in the morning. The hours 
from noon to midnight are marked p.m., the first letters 
of the Latin words post meridiem, "after noon." 

The day used in astronomical work begins and ends 
at noon, 1 twelve hours later than the beginning and 

*■ Why is it most convenient for the civil day to begin at mid- 
night ? Why is it most convenient for the astronomical day to begin 
at noon ? 



106 ASTRONOMY. 



ending of the same civil day. 1 This day is usually 
divided into twenty-four hours, which are numbered 
from 1 to 24. For many purposes an astronomer uses 
the sidereal day, which is about four minutes shorter 
than the mean solar day. 

93. The Week. — This is not a natural astronomical 
division of time, although a very ancient one. The 
names of the seven days of the week were derived as 
follows : Sunday is the sun's day ; Monday, the moon's 
day; Tuesday, Wednesday, Thursday, Friday, and 
Saturday are derived from the names of five old-Eng- 
lish deities. 

94. The Month. — The month is a very ancient di- 
vision of time. At first it lasted from one new moon 
until the next, but this is about twenty-nine and one- 
half days, a number inconvenient in itself and not an 
exact divisor of the year. Presently the year was di- 
vided into twelve months differing somewhat in length. 
The present arrangement of the days in each month 
was made by Augustus, Emperor of Rome at the begin- 
ning of the Christian era. The names of the first six 
months of the year are derived from Roman names, 
mostly deities, July and August are named for the 
Roman emperors Julius Caesar and Augustus, and 
the last four months are named from the Latin words 
meaning seven, eight, nine, and ten, for when these 

1 Thus, July 24, 9 a.m., civil time, would be July 23 d. 21 h., as- 
tronomical time; and July 24, 3 p.m., civil time, would be July 24 
d. 3 h., astronomical time. 

What astronomical times correspond to these civil times ? April 
5, 3 a.m. ; May 10, 12 (noon) ; May 10, 12 (midnight). 

What civil times correspond to these astronomical times ? July 
5 d. 6 h. ; September 8 d. 14J h. j March 3 d. h. 



THE EARTH. 107 



names were given there were but ten months in the 
Roman year. 

95. The Year. — The year which is always used is 
the time that it takes the sun to pass from the vernal 
equinox around to the vernal equinox again. This is 
365 days, 5 hrs., 48 min., 46 sec. 1 These odd hours 
and minutes gave the ancients a great deal of trouble. 
Many devices were used by the different nations of an- 
tiquity to make the different seasons come at the same 
time year after year. 

96. The Julian Calendar. — Julius Caesar found the 
Roman calendar very much in error. Their winter 
months came in autumn, and the 1st of September 
came at the summer solstice. With the aid of an 
Egyptian astronomer he made the ordinary year to 
contain 365 days, but he added one more day to every 
fourth year, and also made the year begin with Jan- 
uary 1. If the year were exactly 365 days and 6 hours 
long, this arrangement would be perfect. But because 
the odd hours and minutes are a little less than one- 
fourth of a day, the Julian years are a little too long, 
and the calendar fell back about 3 days every 400 years. 2 

1 This is called the tropical year, to distinguish it from the sidereal 
year, the time occupied hy the sun in passing from a certain star 
around to that star again. The sidereal year is 21 minutes longer 
than the tropical year, and is, of course, the true year, or period of 
the earth's revolution about the sun. But the tropical year includes 
exactly the four seasons, and is, therefore, more convenient. The dif- 
ference between the two is the result of the precession of the equi- 
noxes (Art. 72). 

2 In the Julian calendar the year is supposed to be exactly 365 J 
days long. This is too great, and if a certain portion of time is di- 
vided into these years there will be fewer years than there ought to be, 
and the count will fall behind ; just as when the foot-rule is too long 
the measurement of a board will be too short. 



108 ASTRONOMY. 



97. The Gregorian Calendar. — In 1582, when Greg- 
ory XIII. was Pope, the calendar had fallen back 10 
days. In that year the vernal equinox came on March 
11, instead of March 21. As the time of Easter 1 and 
other festivals of the Catholic Church depends upon the 
vernal equinox, these festivals were gradually moving 
out of their proper months. To remedy this, the Pope 
introduced the Gregorian Calendar. This simply omitted 
three of the extra days every 400 years. The equinox 
was brought back to its place in the month by drop- 
ping 10 days out of the year 1582, the 5th of October 
being called the 15th. Catholic countries adopted the 
new calendar at once, but it was not adopted in Eng- 
land until 1752, 2 and in Russia the Old Style, as it is 
called, is still in use : so that now in Russia dates 
are 12 days earlier than elsewhere. The leap-years are 
determined by the following simple rule. Every year, 
except the exact centuries, that is divisible by 4 is a leap-year. 
Every exact century that is divisible by 400 is a leap-year. 
Thus, 1884, 1888, 1892, are leap-years, because they are 
divisible by 4 ; 1900 will not be a leap-year, because it 
is not divisible by 400, but 2000 will be a leap-year. 
This calendar loses only one day in about 4000 years. 

98. How to Find what Day of the Week a Given Day 
will be. — A year of 365 days makes 52 weeks and 1 

1 Easter is the Sunday following the first full, moon that occurs after 
the vernal equinox. 

2 By this time the calendar was 11 days behind, for the year 1700 
had intervened. By Act of Parliament in 1752 the day after Septem- 
ber 2 was called September 14. There was great opposition to this 
change, especially among the lower classes. They thought that they 
had been robbed of 11 days, and ran after the members of Parliament 
who had secured the passage of the law, pelting them with stones and 
mud. 



THE EARTH. 109 



day; a leap-year makes 52 weeks and 2 days. Gen- 
erally, then, a given day of the month comes one day 
later in the week each year, except when a 29th of 
February has come between ; then it comes two days 
later. In 1882 the 4th of July is on Tuesday, in 1883 
on Wednesday, but in 1884 on Friday. 

99. Latitude and Longitude. — The latitude of any 
place is its distance in degrees north or south of the 
equator. The longitude of a place is its distance in 
degrees east or west of some fixed meridian. 1 The 
meridian of Greenwich 2 is used more than any other ? 
although different nations use the meridians of their 
capitals also. The location of a place on the earth is 
always determined by its latitude and longitude. And 
it is absolutely essential that a ship-captain should find 
his latitude and longitude when at sea, in order to de- 
termine the course he must take to reach his destina- 
tion and avoid dangers. 

100. How to Find the Latitude of a Place. — With the 
proper astronomical instruments, properly mounted, as 
they are at observatories, it is easy to determine lati- 
tude. The angular distance of a star above the hori- 
zon when it crosses the meridian is measured. As the 
declination of the star is known, a simple arithmetical 



1 What is the latitude of a place on the equator ? On the tropic 
of Cancer? On the Arctic Circle? At the north pole? What is 
the longitude of the north pole, from any meridian? What is the 
greatest possible latitude of any place on the earth ? The greatest 
possible longitude ? 

2 Greenwich (pronounced grin'ij) is close to London, and is the 
seat of the Koyal Observatory of England. American sailors reckon 
from the meridian of Greenwich, and the whole world ought to do it. 
It is mainly national pride that prevents it. 

10 



110 ASTRONOMY. 



solution gives the latitude of the place. 1 At sea, the 
angular height of the sun above the water at noon is 
measured with a sextant (Art. 250), and from this the 
latitude of the ship is found in the same way. 

101. Longitude and Time. — As the earth rotates once 
on its axis in 24 hours, every place on the earth must 
revolve around in a circle in that time. And as every 
circle contains 360°, in one hour every place on the 
earth rotates -^ of 360°, or 15°. 2 This makes the sun 
rise 1 hour later for every 15° that a place is west of us, 
and 1 hour earlier for every 15° that a place is east of 
us. If the sun rises later upon places west of us, of 
course their time is later than ours, — that is, their clocks 
are behind or slower than ours. Places east of us have 
their time, and therefore their clocks, faster than ours. 
The difference of time is, of course, 1 hour for every 
15° that one place is east or west of the other. 3 

102. Longitude Found from the Difference of Time. — 

1 If the star is found to be 70° above the northern horizon, it must 
be 90° — 70°, or 20° farther north of the equator than our zenith, or 
than we are. Suppose a catalogue of stars gives the declination of 
this star as 60° N. Then, as we are 20° nearer the equator, our lati- 
tude must be 40° N. The declination of the sun for every day in the 
year is given in the Nautical Almanac, for the use of sailors. 

2 Does every part of the earth's surface move through the same 
distance in 24 hours ? If there is any difference, which part of the 
earth's surface moves fastest as the earth rotates ? "Which slowest? 

3 A curious effect of this is that messages sent westward by tele- 
graph seem to arrive at their destination before they are sent. The 
difference of time between England and New York is about five hours. 
After the morning papers come out in London, news from them is 
sometimes telegraphed to New York by one of the Atlantic cables 
and printed in our papers the same morning. When Pope Pius IX. 
died in 1878, our American afternoon papers which were printed at 
one o'clock announced that the Pope had died at three o'clock the 
same afternoon. 



THE EARTH. \\\ 



If one had a watch that kept perfect time, he could find 
the longitude of any place exactly. He need only set 
his watch just right at Greenwich, and carry it to the 
place whose longitude is wanted. The difference of 
time between his watch and a clock which gives the 
correct time of the place is the difference of time be- 
tween this place and Greenwich. This multiplied by 
15 gives the difference in degrees, or the longitude of 
the place. If the clock is faster than Greenwich time, 
the longitude of the place is east; if slower, the lon- 
gitude is west. 1 It is impossible to make watches or 
clocks that will keep perfect time, but clocks are made 
which will vary very little. Those made to be carried 
are called chronometers, and are always carried by ships 
at sea. A ship's chronometer keeps Greenwich time 2 
throughout the voyage, and the captain finds the cor- 
rect time at his ship every clear day from observations 
of the sun with his sextant. 3 The difference between 

1 Greenwich time is 5 hrs. 40 sec. faster than Philadelphia time. 
What is the longitude of Philadelphia ? San Francisco time is 3 hrs. 
9 min. slower than Philadelphia time. "What is the longitude of San 
Francisco ? The longitude of Pekin is 116° 27' east : what is the dif- 
ference of time between Pekin and Philadelphia ? When it is noon 
at Pekin, what time is it at Philadelphia? 

2 The chronometer need not have Greenwich time, but the captain 
must know how much too fast or too slow it is. The amount which a 
chronometer gains or loses every day is called its rate, and is carefully 
determined before going to sea. Allowance is made for this in get- 
ting Greenwich time from the chronometer. 

3 Besides the height of the sun above the horizon (which is taken 
for this purpose about 8 or 9 a.m. or 3 or 4 p.m.), the captain needs 
his latitude (got as in Art. 100) and the sun's declination for that day 
(given in his Nautical Almanac). Knowing these, the time can be 
calculated by spherical trigonometry. 

If the days are cloudy, but the nights clear, the moon or certain 
stars or planets may be used. 



112 ASTRONOMY. 



this and the chronometer's time when the observation 
was made gives him the ship's longitude. Thus know- 
ing his latitude and longitude, the captain can find on 
his map just where his ship is. 

103. Longitude Found by Telegraph. — When two places 
are connected by a telegraph-line, this can be used to 
find their difference of longitude in the best and most 
exact way. At the time of the passage of some star 
across the meridian of one place, a signal is sent over 
the wire to the other place. Since the electricity travels 
over the wire at the rate of about 8000 miles a second, 
unless the distance between the places is great/ the 
signal reaches the second place at practically the same 
time it started. If the time when it arrives at the 
second place is observed, the difference of time be- 
tween the two places, and hence the difference of longi- 
tude, is found. When the distance is great enough to 
make the time occupied by the passage of the electricity 
perceptible, a correction is easily made for that. 

104. Change of Days in going around the Earth. — If 
a person travels towards the west, each of his days is 
longer than if he stays in one place. (Why ?) And if 
he travels entirely around the earth in this direction, 
since each of his days has been longer, he has not had 
so many of them, and has had in fact one day less than 
his neighbors who stayed at home. If he has kept an 
account of his days, he will find his reckoning one day 
behind theirs : what he calls Tuesday they call Wed- 
nesday. If he goes around eastward, he will gain a 
day, and his Tuesday will be Monday at his home. It 
is necessary, then, to have some line where the day 
changes. The line now generally used is the one just 
opposite to the meridian of Greenwich, 180° from it 



THE EARTH. H3 



either way. It runs directly north and south, of course, 
through the Pacific Ocean, and nearly through Beh- 
ring's Strait. 1 When voyagers from San Francisco to 
Asia cross this line, they skip a day. If, for instance, 
this meridian is crossed about noon on Monday, the 
rest of that day is called Tuesday. Coming back, a 
day is repeated : Monday noon would suddenly become 
Sunday noon, and the next morning, Monday morning 
over again. 

105. Refraction. — When light passes from a rare trans- 
parent substance to a dense transparent one (as from air 
to water), its course is bent, and it becomes more nearly 
perpendicular to the surface of the dense substance. 2 
But when the light is passing from the dense to the 
rare substance it is bent in the other direction, and be- 
comes more slanting to the dense surface. This bending 
of the light is called refraction. When the end of a tea- 
spoon or an oar is put under water, the part under water 
seems to be bent upward; because when the light 
from that part of the spoon or oar comes out from the 
water into the air it is bent down a little, and, as an 

1 The reckoning in the islands of the Pacific Ocean dees not in all 
cases depend upon their position with respect to this line. Those 
which were settled by voyagers around Cape Horn had calendars one 
day behind those settled by voyagers about the Cape of Good Hope, 
without respect to their situation as regards the 180th meridian. In 
some cases this difference still exists. When our government bought 
Alaska the reckoning there was one day ahead of ours. 

When it is 9 o'clock a.m., Wednesday, at St. Louis (90° 15' W.), 
over what part of the earth is it Wednesday, and what day is it over 
the rest of the earth ? 

Ans.— From 134° 45' E. of Greenwich to 180° W. it is Wednesday. 
Over the rest of the earth it is Thursday (according to Art. 104). 

2 If the light comes down perpendicular to the dense surface it 
cannot become more perpendicular, and so it is not bent at all. 

10* 



114 



ASTRONOMY. 



object always seems to be in the direction in which 
the light comes to the eye, the part under water seems 
to be higher than it really is. Refraction is fully ex- 
plained in Natural Philosophy, and various experiments 
and illustrations of it will be found there. 

106. Refraction of the Heavenly Bodies by the Air. — 
The lower part of the air is denser than the upper part, 
and the light from the heavenly bodies is consequently 
refracted by coming through the air, and they appear 
to be higher up in the sky than they really are. Fig. 
25 illustrates this. is the position of the observer, 




Fig. 25.— The Apparent Elevation of a Star by Atmospheric Refraction. 
(Greatly exaggerated.) 

and Z his zenith. The curved lines represent the at- 
mosphere. The lower ones are closer together, indi- 
cating that the atmosphere there is denser, while the 
higher part of the atmosphere is much rarer, as the 
lines indicate. The true position of the star is at S, 
but its light in passing through the air is bent down, as 
shown in the figure, and the star seems to be at S', above 



THE EARTH. 115 



its true place. As the atmosphere gradually grows 
denser, the path of the light through it is a continued 
curve down to the observer's eye, as shown in the figure. 
The amount of the refraction is not truly represented, 
but greatly exaggerated, to show it more clearly. Re- 
fraction is greatest when the heavenly body is at the 
horizon, for its light is then most inclined, and there- 
fore most bent out of its course. It is then over half 
a degree (35'), but decreases fast at first, then slowly, 
until in the zenith there is no refraction (note 2, p. 113)„ 
In making observations on the height of heavenly 
bodies, astronomers must always correct for refraction. 
That is, they must subtract (Why ?) from the apparent 
height the amount of refraction for that height. This 
is found from tables made for the purpose. 

107. Curious Effects of Refraction. — Since refraction 
elevates heavenly bodies more than J° when they are 
at the horizon, and the sun and moon are each about 
|° in diameter, these two bodies when rising and 
setting seem to be just above the horizon when they 
are really just below it. This makes the sun (or moon) 
rise three or four minutes earlier, and set three or four 
minutes later, than it would if there were no refraction, 
thus adding six or eight minutes to the length of the 
day. 

When just above the horizon, the sun and moon- 
especially the latter — are sometimes seen to be flattened, 
the vertical diameter being shorter than the horizontal 
one. This is also due to refraction. Because the lower 
edge of the sun or moon is nearer the horizon, it is 
elevated by refraction more than the upper edge, thus 
causing the flattening. 

108. Why the Sun and Moon appear Largest when 



116 ASTRONOMY. 



Rising and Setting. — The apparent enlargement of the 
sun and moon when rising and setting is sometimes 
attributed to refraction ; but this is a mistake. This 
enlargement is an optical delusion. When the sun 
and moon are near the horizon they seem larger, be- 
cause the long stretch of country between gives us a 
better appreciation of their great distance from us. 
Every one knows from his own experience that we 
habitually judge of the size of objects from their known 
or suspected distance. Besides, near the horizon we 
can compare the sun and moon with objects whose size 
we know, as fences, trees, houses, and the like. If 
they are looked at through a tube, — a roll of paper, for 
instance, — the illusion will disappear. And if carefully 
measured, the moon's diameter is found to be really 
less at the horizon than when overhead, for at the hori- 
zon it is farther from us. 1 

109. Twilight. — After the sun has set upon us at the 
surface of the earth it still shines for a while upon the 
clouds and air above us. The reflection {not refraction) 
of this light causes twilight. The same cause gives us 
twilight in the morning. As the sun sinks lower and 
lower, less of our sky is lighted up, and the evening 
twilight gradually fades away. Observation shows 
that it does not entirely disappear until the sun is about 
18° below the horizon. Near the equator twilight is 
short, for there the sun always goes down nearly per- 

1 When rising, the moon is farther by the length of the earth's ra- 
dius (How many miles ?) from us than when it is overhead. Let the 
student draw a figure of the earth with the moon upon the horizon, 
and also overhead, and explain this clearly. The sun is so far away 
that the difference between its morning and its noon diameter could 
not be detected by measurement. 



THE EARTH. 117 



pendicular to the horizon. But in the temperate and 
frigid zones the sun always moves obliquely to the 
horizon at sunset and sunrise, and must move more 
than 18° in its path to go 18° below the horizon. The 
nearer we go to the poles, the more oblique is the sun's 
motion, and the longer twilight lasts. 1 In Northern 
Europe twilight in midsummer lasts all night, and at 
the north pole it lasts more than two months. 

110. The Aurora Borealis. — The aurora borealis, 2 or 
simply the aurora, is in some years a frequent phe- 
nomenon in our northern skies. It commonly consists 
of rays of light, sometimes of a reddish tinge, extend- 
ing up above the northern horizon. It is more com- 
mon the farther north one goes. But it is more fre- 
quently seen around the Arctic Circle and the magnetic 
pole than near the north pole itself. Many attempts 
have been made to measure the height of the aurora, 
and the results vary from a few thousand feet to five or 
six hundred miles. 

How the aurora is produced is not yet known. It is 
probably caused in some way by electricity, for auroral 
displays are very frequently accompanied by electric 
storms upon the earth ; strong currents of electricity 

1 This may be very clearly shown with any globe which has a hori- 
zon (a horizontal ring about the middle). For a place in the north- 
ern hemisphere, slide the globe around until the north pole is as many 
degrees above the horizontal ring as the place is north of the equator. 
Now mark with chalk the probable place of the sun at that time of 
year (on a celestial globe the place is marked), and revolve the globe. 
(Which way?) If the place is far north, the pole will be near the 
horizon, and the chalk-mark will be seen to turn through a long arc 
before it is as much as 18° below the horizon. 

2 Aurora borea'lis, from Latin aurora, daybreak, and borealis, 
northern. 



118 ASTRONOMY. 



pass along telegraph-wires, and compass-needles are 
much disturbed. And, besides, if electricity be allowed 
to pass through a long glass tube from which the air 
has been almost exhausted with an air-pump, an ap- 
pearance very much like the aurora is produced. 

At certain periods displays of the aurora are unusu- 
ally frequent and brilliant. As has been said in Art. 
42, these periods seem to occur about every eleven 
years, and at the same times as the periods of numer- 
ous sun-spots, with which they are probably in some 
unknown way connected. For several years before 
1881 the auroras were very infrequent, but during 1881, 
1882, and 1883 they increased in number and brilliancy, 
and then the number gradually diminished for several 
succeeding years. Records and descriptions of auroral 
displays would be well worth making. 



The Tides. 

111. What the Tides are. — Every one who has spent 
even a short time on the sea-coast, on a bay, or near the 
mouth of a river, has noticed that every day for about 
six hours the water slowly rises (flood tide) until it is 
several feet deeper, and then as slowly falls (ebb tide) for 
the next six hours. The same thing is repeated in the 
night. These risings of the waters of the ocean are 
called tides. They are caused by two waves which are 
constantly passing around the earth from east to west, 
opposite to the direction of the earth's rotation. They 
are not exactly twelve hours, but nearly twelve and one- 
half hours apart, so that each of the two tides comes 
about one hour later every day. 

112. Tides caused mainly by the Moon. — If the matter 



THE EARTH. 119 



be looked into a little more closely, it will be noticed 
that high tide always comes when the moon is about 
the same place in the sky. 1 The moon rises about one 
hour later every night, and we have seen that high tide 
comes about one hour later every day. This remark- 
able connection between the moon and the tides was 
noticed in very ancient times, and men knew that the 
moon in some way caused the tides, long before they 
could explain how it caused them. Sir Isaac Newton 
was the first to show just how they were caused by the 
moon. 

113. How the Moon causes the Tides. — To show ex- 
actly how the moon causes the tides requires a difficult 
mathematical demonstration. But the common expla- 
nation is illustrated by Fig. 26. Because the water 




© 



Fig. 26.— The Tides. High tide at and D. Low tide at the two points half-way be- 
tween, immediately in front of and behind the middle of the figure. 

on the side of the earth nearest to the moon is more 
strongly attracted by the moon than the earth itself is 
attracted by it, the water is heaped up on that side, 
forming the direct tide, at D in the figure. And because 

1 This is true of either of the two daily tides, although only one of 
*hem would occur while the moon was above our horizon. At New 
York high tide always occurs when the moon is about southeast ; at 
New Castle, Del., when the moon is south ; and at Baltimore when 
the moon is rising and setting. Why these intervals differ at differ- 
ent places is explained in Art. 115. 



120 ASTRONOMY. 



the moon attracts the earth itself more strongly than it 
attracts the water on the opposite side of the earth, it 
pulls the earth away from the water there, leaving it 
heaped up on the opposite side, and forming the oppo- 
site tide there, at in the figure. This explanation 
shows pretty clearly how the direct tide is formed ; but 
most persons cannot see how the earth can be pulled 
away so as to leave the opposite tide behind it and yet 
approach no nearer to the moon. The following ex- 
planation may help to make the cause of the opposite 
tide clear. 

114. How the Opposite Tide is produced. — "We are accus- 
tomed to say that the moon revolves about the earth. 
But it is proved in Natural Philosophy that it is impos- 
sible for one body to stand still while another revolves 
about it in this way. They must both revolve about their 
common centre of gravity} The centre of gravity of the 
earth and moon is within the earth, about three-fourths 
of the way from the centre to the surface, at C in the 
figure, so that the earth as well as the moon is really 
revolving about this point C. As the earth revolves 
around this point, the water at 0, being attracted 
less strongly by the moon, swings out into a little larger 
circle. This heaps up the water there, and always 
makes a tide on the opposite side from the moon. If 
a hollow india-rubber ball be pulled on its two opposite 
sides by two strings, it will take this spheroidal shape. 
The opposite forces in the case of the earth are the 
moon's attraction and centrifugal force. The land is 
solid and cannot be pulled out of shape, but the water 



1 The centre of gravity of two bodies is the point about which they 
would balance each other. 



THE EARTH. 121 



yields. The revolution of the earth and moon around 
their common centre of gravity takes about a month 
(27J days). The reason we have a tide about every 
twelve hours is that the earth in rotating on its axis 
turns under these two projections and carries us around 
to them. It is this turning of the earth under these two 
projections that causes the tidal friction which it is 
thought may be slowly retarding the earth's rotation 
(Art. 68). This also explains the fact that the main 
motion of the tidal waves is from east to west, for we 
are carried towards the east to them. 

115. The Sun's Influence upon the Tides.— Although 
the sun is so much farther than the moon from the 
earth, yet its prodigious mass makes its attraction far 
greater upon the earth than the moon's attraction. But 
the power to raise a tide depends not so much upon the 
strength of the attracting force as upon the difference of 
its attractions upon the opposite sides of the earth. 
The sun is so far away that it draws the opposite side 
of the earth almost as strongly as the near side, so that 
it does not draw up a high direct tide, nor does it draw 
the earth much away from the opposite waters to raise 
an opposite tide. Yet the sun's influence is perceptible. 
The sun can raise tides about two-fifths of the height of 
the moon's tides, but these are generally combined with 
those raised by the moon. If, in Fig. 26, the sun were 
to the right of M (new moon), the sun's tides would be 
piled upon the moon's tides, which would make them 
unusually high. The same result would follow if the 
sun were opposite to the moon (full moon). These are 
the spring tides, and occur every two weeks. But if 
the sun and moon are 90° apart, as when the sun is 
directly in front of or behind the earth, in Fig. 26, 

11 



122 ASTRONOMY. 



they pull the water in opposing directions. Then the 
sun lowers the moon's tides : these are called neap tides. 
The difference between spring and neap tides at New 
York is about two feet. 

116. The Land modifies the Action of the Tides. — If the 
whole earth were covered with water of the same depth, 
the tides would pass regularly around the earth; 1 but 
continents and shallow water greatly modify the action 
of the tides. Since the Antarctic Ocean is the only one 
extending around the earth, the tidal waves are be- 
lieved to originate there. Branches of these run up 
the great oceans opening into the Antarctic, and these 
run into our harbors and bays, causing the tides there. 
As the tide at the mouth of a river rises, it presently 
becomes higher than the water farther up the river, 
and, as it must flow down-hill, the water rushes up the 
river and causes the tides there. Along the lower part 
of rivers emptying into the ocean, while the tide is 
rising a current runs up the river. At high tide the 
current turns and begins to run down again towards 
the sea. But for some distance from the mouth the 
current runs down for more than half of the twelve 
hours, for it is some time after high tide begins before 
the water rises to the ordinary height of the river there. 

117. Effect of Bays upon the Height of the Tide. — If a 
bay has a broad mouth opening in the direction of the 

1 In the deep waters of the ocean the tide-wave, like other water- 
waves, moves forward by the rising and falling of the particles of 
water, and not by their moving forward. The motion of each par- 
ticle forward and backward is very slight; just as when a breeze 
sweeps across a wheat-field, a wave passes swiftly over the field, but 
each head of wheat only bends over and rises again. Near a shore, 
where the water grows shallow, the tide may have considerable for- 
ward motion. 






THE EARTH. 123 



tidal wave, and gradually becomes narrow towards its 
upper end, it acts like a funnel, and the water may 
be forced up to a great height there. This accounts 
for the different heights of the tide at various places 
along the coast. In mid-ocean the average height of 
the tides is about three and a half feet. At New York 
it is four and a half feet; at Boston, more than twice 
as great. At the upper end of the Bay of Fundy, and 
in the English Channel, the tides sometimes rise to a 
height of seventy feet. 

118. Tides on Inland Seas and Lakes. — Seas which 
have little or no communication with the ocean have 
very little tide. On the Mediterranean, much the 
largest of these seas, there is a tide about eighteen 
inches high, which must be raised upon the sea itself, 
for the narrow Strait of Gibraltar allows the ocean 
tides to affect but a small part of the sea. On the 
great American lakes a tide one or two inches high 
has been detected. 



124 ASTRONOMY. 



CHAPTER V, 



THE MOON. » 



Distance from the Earth, 240,000 Miles. Diameter, 2,160 
Miles. Length of Year, the same as that of the Earth. 
Length of Day, 29 J Days. Specific Gravity, 3 J. 

119. The Moon a Satellite of the Earth — The moon 
revolves about the earth 1 from west to east, just as the 
earth revolves about the sun, but makes a revolution 
every 27J days. Every one must have noticed this 
motion of the moon among the stars towards the east 
from night to night. 2 The new moon is first seen in the 
early evening, low in the west At the same hour the 
next evening it is higher up in the sky : it has moved 
eastward among the stars since the night before. The next 
evening it is higher still, and in two weeks it is in the 
opposite side of the heavens, and is seen in the early 
evening just rising in the east. This motion of the 
moon causes it to rise nearly an hour later every night, 
— a familiar fact, and one often useful in determining 
off-hand a few days in advance how early in the even- 
ing there will be moonlight. 

1 Keally both earth and moon revolve about their common centre of 
gravity (Art. 113), but, as their centre of gravity is inside of the earth, 
the statement here is correct. 

2 Be careful again to distinguish this motion from the nightly 
motion from east to west caused by the earth's rotation. 






TEE MOON. 125 



120. The Moon's Orbit around the Earth. — The moon's 
orbit around the earth, like the orbits of all the planets 
around the sun, is an ellipse. Its eccentricity (Art. 25) 
is about -^ : four times that of the earth's orbit. This 
makes the moon's distance from the earth vary nearly 
40,000 miles ; but the eye could not distinguish its orbit 
from a circle, although it could easily see that the earth 
is not in the centre of the orbit. The point in the 
moon's orbit which is nearest to the earth is called the 
perigee ; l the farthest point is the apogee. 2 

121. The Moon's Path around the Sim. — While the 
moon is revolving about the earth, the earth itself is 
moving forward more than 1J millions of miles a day 
in its own orbit around the sun. This makes the 
moon's real path a waving line, crossing the earth's 
orbit backwards and forwards. Fig. 27 shows this. 




Fig. 27. — The Dotted Line, Part of the Earth's Path around the Sun. The 
Full Line, Part of the Moon's Path around the Sun. 

When the earth is at E, the moon is just in front of it, 
at M. But the earth by its attraction holds the moon 
back, and gradually gains upon it, until, one week later, 
at E 2 and M 2 , they are side by side. The earth's 

1 Per'i-gee, from Greek peri, near, and ge, the earth. 

2 Ap'o-gee, from Greek apo, from, and ge, the earth. (G is soft 
in both words.) What points of the earth's orbit correspond to these ? 

11* 



126 ASTRONOMY. 



swifter motion carries it on past the moon, and in 
another week it is at E 4 , just in front of the moon. In 
these two weeks the moon seems to us to have made 
half a revolution around the earth ; really it has moved 
along the curve MM 4 , inside of the earth's path, and 
somewhat slower than the earth, so that from being 
immediately in front of the earth it has fallen back 
and is now right behind it. Here it crosses the earth's 
path, and, pulled on by the earth's attraction, gains 
upon it, and in another week is beside it again, at 
M 6 ; and at the end of the month it is immediately 
in front of the earth again, and crosses its path. It has 
now, as it seems to us, completed a revolution around 
the earth, and so it has. It is just as if a person while 
on the deck of a fast-sailing ship should walk slowly 
around the mast. He really does walk around the 
mast, but on account of the swifter motion of the ship 
his path over the surface of the earth is a wavy line 
crossing the path of the ship's mast. 

The figure necessarily exaggerates the amount of this 
wavy motion : so small is it in comparison with the size 
of the earth's orbit, that the moon's path could scarcely 
be distinguished from a perfect ellipse. And if the 
earth were suddenly destroyed, the moon would keep 
on in a perfect ellipse about the sun, which could not 
easily be distinguished from its present path. 

122. The Moon's Phases. — The most noticeable fea- 
ture of the moon is its varying phases, from the thinnest 
crescent to the round full moon, and then back to the 
crescent again. To understand these phases, it must be 
remembered that the moon shines only by reflecting 
sunlight, and therefore the half of the moon which is 
turned towards the sun is always bright, while the other 



THE MOON. 127 



half is always dark. When the moon passes between 
the earth and the sun, 1 its dark side is turned towards 
us. This is new moon, but we cannot see it then. A 
day or two later, it has moved to one side of the line 
between the sun and the earth, so that we can see a 
narrow strip of the bright side of the moon looking 
like a thin crescent. 2 We call this the new moon ; but 
the exact time of new moon as given in the almanac 
was when the sun and moon were in conjunction (Art. 
24). In a week the moon is around beside the earth, so 
that of the side turned towards us half is light and half 
is dark. The moon is half full : it is at the first quarter. 
Then it becomes gibbous, or more than half full ; in a 
week from half full it is on the opposite side of the earth 
from the sun, and we see the whole surface lighted up by 
the sun : it is full moon. 3 In another week the moon is 
beside us again : it is again half full, and is at the third 
quarter. As it goes around towards the sun, more and 
more of its dark side turns towards us, and the crescent 
grows narrower and narrower, until it disappears at 
new moon again. 

Fig. 28 illustrates these changes. The sun is sup- 

1 The moon does not generally pass directly between the earth and 
the sun. but a little above or a little below it. When it does pass 
directly between, we have an eclipse of the sun. Nor at full moon 
are the three bodies generally in a straight line. "When they are, 
the moon is eclipsed. 

2 The horns of the new and of the old moon always point away 
from the sun. . Do you see why ? 

There is a wide-spread superstition that if the horns of the new 
moon point upward there will be dry weather, because the moon 
holds the water, but that if the horns point dovmward there will soon 
be rain. What do you think about it ? (See Art. 135.) 

3 What time of day, and where in the sky, do we see the new moon ? 
the full moon ? the old moon ? Why ? 



128 



ASTRONOMY. 



posed to be above the figure. The figures upon the 
circumference of the circle represent the moon in its 




Fig. 28.— The Moon's Phases. 



different positions : the half turned towards the sun will 
be noticed to be always bright. The outside figures 
show the different phases. 



THE MOON. 129 



123. Time in which the Moon Revolves around the 
Earth. — The moon makes a revolution around the earth 
in 27J days. That is, if the moon passes a certain star 
at twelve o'clock to-night, it will pass the same star 
again 27J days afterwards. But if the moon passes the 
sun at a certain time (new moon), when it comes around 
to the same place the sun has moved forward in its ap- 
parent yearly motion around the earth, and it takes the 
moon more than two days longer to catch the sun. So 
that the time from new moon to new moon is 29J days. 
This is called a lunation, 1 or lunar 1 month. It is the 
foundation of our months. 

124. The Same Side of the Moon always turned towards 
the Earth. — A little attention shows that we always see 
the same side of the moon. The well-known dark 
markings upon it are always seen w T hen enough of the 
side next us is lighted up to show them. Closer ex- 
amination with the telescope proves the same thing. 
The moon must therefore turn upon its axis just as fast 
as it revolves around the earth, 2 or in 27J days. This 
is the moon's sidereal day. Its solar or real day is the 
time from new moon until new r moon again, 29J days. 
Daylight and night on the moon are each, then, nearly 
fifteen days in length. 

A curious and quite probable cause of this remark- 
able motion of the moon is this. The present appear- 
ance of the moon's surface gives strong evidence of 
the fact that the moon was once at least partly liquid 
or molten. The earth's attraction must then have 

1 From Latin luna, the moon. 

* Let the student walk around a table, always keeping his face 
towards it, and he will find that his body has turned completely 
around, so that he has faced every side of the room in succession. 



130 ASTRONOMY. 



raised enormous tides in this molten surface, and 
the friction of these tidal waves may have retarded 
its rotation until it turned upon its axis just as it re- 
volved about the earth. We have already learned that 
the tides upon the earth may be retarding its rotation 
slightly. 

125. The Moon's Librations. — Although the moon 
always keeps the same side turned towards the earth, 
still there are some small oscillations that allow us 
to see a small part of the other side along the edge 
These oscillations are called librations} There are three 
of these. The moon turns on its axis uniformly, but 
since its path around the earth is an ellipse, its motion 
in that path is not uniform (Art. 29). On this account 
the moon sometimes turns on its axis a little faster or 
a little slower than it revolves about us, and allows us 
to see somewhat farther than usual along its equator. 
If the moon's path were a circle, there would be no 
such libration as this. 

A second libration is due to the fact that the moon's 
axis is not quite perpendicular to the plane of its orbit, 
and its poles therefore lean towards the earth alter- 
nately, just as the earth's poles lean towards the sun 
to produce summer and winter. This allows us to see 
at every revolution of the moon a little way beyond its 
poles. 

And, finally, two observers on opposite sides of the 
earth can each see farther around the moon in his di- 
rection than the other can, so that both together see 
more than half of the moon's surface. This is the 
third libration. 

1 From Latin libratio, a balancing. 



THE MOON. 131 



These three librations together enable us to see at 
various times nearly one-fifth of the opposite side of 
the moon, so that in all about six-tenths of the moon's 
surface can be seen by us. 

126. The Harvest Moon. — A celestial globe shows 
very clearly that near the autumnal equinox (Septem- 
ber 22) the ecliptic (Art. 30) is most inclined to the 
horizon — that is, it makes the smallest possible angle 
with it — about sunset. And it is also about sunset 




Fig. 29. — The Harvest Moon. 

that the full moon rises. The figure shows the full 
moon just on the horizon at sunset on September 
22. At the time of sunset on September 23 the moon 
has moved its regular daily journey of 13° along the 
ecliptic 1 to the position shown in the figure. It is now 
below the horizon, but, because the ecliptic makes so 

1 The moon's path does not quite coincide with the ecliptic, but 
does so very nearly, never being more than about 5° from it. 



132 ASTRONOMY. 



small an angle with the horizon, the moon is but a 
short distance below it, and will rise at a only a few 
minutes later than on the night before. By the next 
evening the moon is another 13° farther along on 
the ecliptic, but still it is not far below the horizon, 
and will rise at b a few minutes later again. The next 
night it will rise at c a few minutes later still. And so 
the full moon which comes on, or nearest to, the au- # 
tumnal equinox rises only a few minutes later evening 
after evening for several days. 1 This is called the har- 
vest moon, because in England harvest comes at this 
time, and the moon being about full, and rising night 
after night nearly at the same time, helps with its light 
in the gathering of the harvest. The harvest moon is 
much more noticeable in England and other high lati- 
tudes than here. In Edinburgh, if the moon rose at 
six o'clock on September 22, it would rise only fifteen 
minutes later % the next night, and only fifteen min- 
utes later again on the next. In the northern United 
States the harvest moon rises about half an hour later 
each night. 

127. Observation of the Moon. — To the naked eye the 
moon's surface shows only the dark patches which 
vivid imaginations sometimes call " the man in the 
moon." These patches are simply parts of the moon's 
surface which are darker than the rest. But through 
a telescope its appearance is very different. The moon 

1 The figure will probably make the explanation of the harvest 
moon clear to the student, but a globe having the ecliptic marked 
upon it is better than any figure. If the successive daily positions 
of the moon be marked upon the ecliptic with chalk, beginning at the 
equinox, when the globe is revolved these marks will appear at the 
horizon only a short time apart. 



THE MO OK 133 



is much nearer to us than any other heavenly body, 
and for small telescopes it is therefore one of the most 
interesting and beautiful objects in the heavens. The 




Fig. 30. — The Moos near the Last Quarter. (From Newcomb's Popular Astronomy.) 

best time for evening observation of the moon is from 
the time it is new until it is half full. The inner edge 

12 



134 



ASTRONOMY. 



of the crescent, separating the light and dark parts of 
the moon, called the terminator^ will be very uneven and 




Fig. 31.— A Group of Lunar Mountains. 



jagged. This is caused by the rough mountainous sur- 
face of the moon. Near the terminator, round pits, re- 



THE MO OK 135 



sembling pock-marks, will probably be seen : these are 
the great crater mountains. It is their dark shadows 
beside them that make all these mountains and craters 
distinct. At full moon these shadows disappear, for the 
sun is then shining directly down upon them. The 




Fig. 32.— The Lunab Mountain, Copernicus. 

bright rays or streaks that run out in all directions from 
some of the mountains constitute the most interesting 
feature of the full moon. 

128. The Geography of the Moon. — The dark patches 
on the moon's surface were formerly supposed to be 



136 ASTRONOMY. 



seas, and were accordingly named as the different 
seas on the earth are named ; and, although it is now 
known that there is no water on the moon (Art. 130), 
they are still called seas. They seem to be great 
plains, like our prairies. The moon's surface is very 
mountainous. By measuring the length of their shad- 
ows the height of many of these mountains has been 
calculated. Some of them are over 25,000 feet high, 
nearly or quite as high as any upon the earth, although 
the moon is so much smaller than the earth. The 
larger mountains have been named after eminent sci- 
entific men. 

129. The Crater Mountains are the most curious fea- 
ture of the moon. They are saucer-like depressions 
surrounded by ring-mountains, and resemble the cra- 
ters of our volcanoes, but many of them are on a much 
larger scale. Sharp 'peaks often rise from the middle 
of the craters, like the cones in our volcanic craters. 
Fig. 31 shows a part of the moon's surface covered 
with such craters of all sizes, while Fig. 32 shows a 
single one of the largest of these crater mountains. 
The diameter of this crater is about fifty miles. The 
bright rays running out from the mountains seem to 
be great cracks which have filled up with a whiter sort 
of rock. 

The moon's surface has been carefully observed by 
astronomers, and very accurate maps of it have been 
drawn and published. 1 We undoubtedly have a much 

1 The best map of the moon yet published is by two German astron- 
omers, Beer and Maedler. It is two and a half feet in diameter. Dr. 
Schmidt, of Athens, Greece, has made one nearly eight feet in diam- 
eter, which has been published at the expense of the German Gov- 
ernment. 



THE MO OK 137 



better knowledge of the outline features of the side of 
the moon which we can see than of the great part of 
the earth's surface. Magnificent photographs l of the 
moon have also been taken, which picture with won- 
derful beauty and accuracy the moon's surface as seen 
through a telescope. Fig. 30 is from a photograph by 
Prof. Henry Draper. 

130. No Air or Water on the Moon. — There is no re- 
liable evidence of either air or water upon the moon. 
The moon in its monthly journey around the earth 
frequently passes between us and a star. If the moon 
had an atmosphere, the star's light would be refracted 
by it, just as it goes behind the edge of the moon. 
This refraction would displace the star slightly, just as 
refraction by our atmosphere does (Art. 105). But the 
most careful observations of these occultations? as they 
are called, fail to show any such displacement. This does 
not prove that the moon is absolutely without atmos- 
phere, but does prove that if there is any it must be 
very insignificant. Besides, if the moon had an at- 
mosphere of any extent, we should expect it to make 
the moon's surface somewhat indistinct, and also to 
cause a twilight or sort of shading along the edge of 
the bright part of the moon. But nothing of the kind 
can be noticed. The inequalities of the moon's sur- 
face stand out with the utmost sharpness and distinct- 

1 The best photographs of the moon yet taken are by Mr. Kuther- 
furd, of New York. 

Mr. Henry Harrison, of New York, is publishing six very accurate 
and beautiful colored representations of the moon at different phases. 
The diameter of the moon is eighteen inches, and all the features are 
distinctly given. The colors are such as are seen in the telescope. 

2 Occulta / tion, from Latin occultatio, a hiding. 

12* 



138 ASTRONOMY. 



ness, and the terminator, the boundary-line between 
day and night, is perfectly defined, giving no evidence 
of a twilight. Observations with the spectroscope 
(p. 298) also prove that there is no considerable atmos- 
phere on the moon. 

If there w r ere water upon the moon, it would evap- 
orate and form an atmosphere of vapor as easily de- 
tected as one of air. The visionary idea has been ad- 
vanced that the moon's centre of gravity is so much 
nearer the other side of the moon that all the air and 
water have run around to the opposite side, which we 
never see. But there is no evidence of anything of the 
kind. In all probability, the unseen side of the moon 
is very much like the side we do see. 

131. Is the Moon Inhabited? — The absence of air and 
water at once answers this question in the negative. 
Besides, changes of heat and cold upon the moon must 
be so extreme as to destroy any sort of living beings 
that we know anything of. Two weeks of constant 
sunshine, unchecked by any atmosphere or the slight- 
est cloud, would be an extremely uncomfortable blaze, 
not necessarily, however, raising the temperature very 
high, for we know that snow can exist under some- 
what similar circumstances on our high mountains. 
During the two weeks of night which follow an 
intense cold would prevail, carrying down the tem- 
perature probably much below the lowest records of 
our polar regions. We may therefore assume that 
life, such as is represented by the common terrestrial 
animals and plants, does not exist on the moon. 

132. The Moon's Past and Present. — The moon gives 
strong evidence of having been at one time molten. 
The great craters were probably formed then by the 



THE MO OK 139 



bursting forth of great bubbles of gas from the inte- 
rior. There is also clear evidence of volcanic action 
there in the past. But there is no trustworthy evidence 
that any such volcanic action has ever been seen by 
men. The moon's volcanoes have probably been long 
extinct. There has, indeed, been considerable evidence 
of change in the appearance of some small portions of 
the moon's surface in late years, but the matter is still 
in doubt. In order to account for the absence of air 
and water upon the moon, it has been suggested that, 
as the moon cooled off, the contraction in its interior 
caused great caverns there, which have swallowed up 
its atmosphere and oceans. The idea is ingenious and 
not improbable, but of course nothing is really known 
about it. Whether the moon ever contained life we 
cannot say, but it is now a dead, sterile mass of rock. 

133. The Occultation of Stars by the Moon. — An oc- 
cultation of a star by the moon is an interesting and 
at first a surprising sight. Small stars are frequently 
occulted, the larger ones and the planets more rarely. 
The moon's brightness overpowers the light of all but the 
brightest stars as they come to its edge, so that a tele- 
scope is needed to watch their occultation. If the star 
disappears at the dark edge of the moon, which is always 
the case if the occultation comes before full moon, the 
phenomenon is very striking. The star disappears sud- 
denly and apparently without there being anything to 
cause its disappearance. Its reappearance on the other 
side is just as sudden. This proves that the stars have 
no apparent size whatever, but are mere points of light 
(Art. 204), and also furnishes a further proof of the ab- 
sence of atmosphere or vapor on the moon, which would 
make it fainter just before its disappearance. An occul- 



140 



ASTRONOMY. 




tation of one of the prominent planets is rather rare, 
and always attracts attention. As in the telescope these 
show disks of considerable size, they disappear gradu- 
ally. Fig. 33 shows an occupation of Jupiter. All the 
occultations of stars bright enough to be seen by the 

naked eye are predicted 
every year in the Nau- 
tical Almanac, 1 for the 
use of observers. Stu- 
dents having the use of 
telescopes should be on 
the lookout for them. 

134. Light and Heat 
from the Moon. — Several 
astronomers have tried to 
determine the amount of 
light shed by the moon. 
The results are generally expressed by comparing the 
moon's light with that of the sun. The latest and most 
reliable result is, that the sun gives 600,000 times as 
much light as the full moon. Chambers aptly says 
that " if the whole sky were covered with full moons 
they would scarcely make daylight." 

Until recently, not the slightest heat from the moon 
could be detected. But by concentrating its heat with 
the largest telescopes, and using the most delicate ap- 
paratus, it has finally been detected and measured. It is 
found that the moon's heat would raise the mercury in a 
thermometer only about 5 ^ - of a degree Fahrenheit. 2 

1 See foot-note on page 169. 

2 So slight a rise in temperature could not be noticed in any ther- 
mometer, even if the moon's- heat were concentrated upon it by the 
largest telescope in the world. The Thermo-electric Pile, by which 



Fig. 33. — An Occultation of Jupiteb. 



THE MOON. 141 



So that, while the moon's light is a great advantage to 
us, its heat does nothing to warm us. 

135. The Moon and the Weather. — It is very com- 
monly believed that the moon exercises great influence 
upon the weather. This is a mere superstition. No 
good reason for this supposed influence has ever been 
given, and accurate records of the weather kept for 
many years show that not the least reliance can be 
placed upon the moon's " weather signs." It is true 
that various scientific men have attempted to prove 
that there is on the average a slight difference in the 
rainfall at different phases of the moon. But, unfor- 
tunately, they seem to come to directly opposite about 
as often as to the same conclusions ; and the greatest 
difference thus claimed by any of them is so slight that 
ordinary observation would never notice it at all. So 
that we may fairly conclude that any such differences in 
the amount of rainfall at different ages of the moon are 
simply accidental, and will, as likely as not, be reversed 
during the next period of equal length. 1 

The best-supported of these theories is that the heat 
of the full moon does something to clear away clouds. 
If the moon has any such effect, — which is doubtful, — 
it must be inconsiderable. The notion that the signs 
of the moon in which they were planted influence the 

the slightest heat starts a current of electricity, was used in the ex- 
periments. 

1 The conspicuous failures of many reputed weather prophets should 
convince people of the absolute impossibility in the present state of 
science of foretelling the weather for any particular day more than a 
day or two in advance. 

It is scarcely necessary to remark that the weather prognostications 
so often to be found in our common almanacs have not the slightest 
value. 



142 ASTRONOMY. 



growing of the crops is still extremely prevalent, es- 
pecially among farmers. This is still more absurd than 
the belief in the moon's influence upon the weather. 
There is nothing in reason or in facts to warrant any 
such beliefs. They are as foolish, if not so hurtful, as 
was our forefathers' belief in witches. 

136. Appearance of the Earth from the Moon. — From 
the moon the earth would seem to be a splendid large 
moon, having nearly four times the diameter and thir- 
teen times the surface of our moon. The earth would 
go through its phases just as the moon does for us, but 
these phases would be exactly opposite to those of the 
moon. When the moon is new the earth w r ould be full, 
and while the moon increases from new to full the 
earth would decrease from full to new. 

Every one has noticed that when the moon is only a 
few days old the dark part is faintly lighted up, and 
the whole moon can be distinctly seen. This is often 
called " the old moon in the new moon's arms." It is 
simply the earth-shine lighting up the moon's surface, 
or, as it might fairly be called, moonlight on the moon. 
It can be seen as well before as after new moon. 1 

Since the same side of the moon is always turned 
towards us, the earth to an observer on the moon would 
never rise or set. At any one place on the moon it 
would always be seen in the same place in the sky, 2 

1 Why should it be seen about the time of new moon ? 

3 Although the earth would stay at about the same place in the sky, 
it would not stay at the same place among the stars. They would 
rise and set as they do for us, but would take two weeks instead of 
twelve hours to pass from east over to west. 

Because there is no atmosphere to reflect the sun's rays everywhere, 
and thus overpower their light, the stars could be seen upon the moon 
day and night, and the sky itself would always be intensely black. 



THE MOON. 143 



subject only to slight oscillations from the librations. 
And there it would go through all its phases, growing 
from new to full and waning back to new again. To 
an observer about the middle of our side of the moon 
the earth would be always overhead. To one near the 
edge it would always be on the horizon, while on the 
opposite side it would never be seen. 

Notwithstanding its size, it is not likely that the fea- 
tures of the earth's surface could be distinctly seen from 
the moon. Our atmosphere, by reflecting the sun's 
light, and by obstructing the light coming through it 
from the earth, would always produce an indistinct- 
ness, and clouds would of course completely hide every- 
thing beneath them. It is probable that the earth is 
a better reflector of light than the moon, and would 
therefore be more brilliant as well as larger than the 
moon. 



144 ASTRONOMY. 



CHAPTER VI. 

ECLIPSES. 

137. Causes of Eclipses. — The moon is eclipsed by 
passing into the earth's shadow. As the moon shines 
by reflecting sunlight, and the earth then cuts off 
this sunlight, the moon is of course darkeped. When 
this occurs, the moon must be on the opposite side of 
the earth from the sun : so that an eclipse of the moon 
can occur only at full moon. The sun is eclipsed when 
the moon passes directly between the sun and the earth. 
The sun and the moon must then be on the same side 
of the earth. An eclipse of the sun, therefore, can occur 
only at new moon. 

138. Why Eclipses do not occur at every New and Full 
Moon. — If the moon's orbit were in the plane of the 
earth's orbit, as it seems to be in Fig. 34, we should 
have an eclipse of the sun at every new moon, for it 
would always pass then between the earth and the sun, 
and at every full moon we should have an eclipse of the 
moon. But the moon's orbit is inclined to the earth's 
orbit at a small angle (about 5°). It is just as if one 
were to take hold of the moon's orbit at (Fig. 34) 
and lift that side of it up a little way, letting it swing 
about an axis from A to B. The side T would be as 
far below the page as O is above it. The page on 
which the rest of the figure still lies represents the 
plane of the ecliptic or earth's orbit. The points A 



ECLIPSES. 145 



and B would be the moon's nodes (Art. 32). Because 
of this inclination of its orbit, the moon in passing 
around the earth generally passes a little way above or 
below the sun and the earth's shadow, and so there 
is no eclipse. 

139. The Eclipse Seasons. — "When the earth is on or 
near the line along which the two orbits intersect, the 
axis AB about which we turned the moon's orbit in 
the last article, then one or more eclipses will occur. 
The earth in passing around its orbit must cross this 
line twice a year. 1 The eclipses, therefore, are all 
grouped together in two seasons, nearly six months 
apart. They can only occur within seventeen or eigh- 
teen days of the date when the earth crosses this line, 
called the line of the moon's nodes. The eclipse sea- 
sons do not occur at the same dates each year, but 
move forward about nineteen clays. That is, if eclipses 
take place on or near January 1st, the next time will 
be about June 21st, and the next about December 
12th. 

140. Shadows cast by the Moon and the Earth. — Be- 
cause the sun is larger than the earth or the moon, the 
shadows cast by these bodies are cones ; they taper off 

1 A little thought will make this clear. The earth in Fig. 34 
moves around the sun in a direction opposite to the motion of the 
hands of a watch, carrying the revolving moon along with it. The 
line between the earth and the sun will at first run above the T side 
of the moon's orbit. As we look at the figure, the new moon now 
passes above the line between the earth and the sun, and the full moon 
under the shadow, causing no eclipses. In about six months the earth 
is at the line of nodes again, and the eclipses occur. For the next 
six months the line between the sun and the earth is below the O side 
of the moon's orbit, and the new moon passes under and the full 
moon above, causing no eclipses as before. 

13 



146 ASTRONOMY. 



to a point. The earth's shadow is of course larger and 
longer than the moon's. Fig. 34 shows parts of these 
shadows. To an observer in either of these shadows 
the sun is entirely hidden. 

If one were between the earth's shadow and the line 
MC or ND, it is evident that the earth would hide a 
part of the sun. This space around the shadow is 
called the penumbra. The nearer one is to the edge of 
the shadow the greater is the part of the sun hidden, 
and the less light there is. The penumbra does not 
grow narrower and finally come to an end, like the 
shadow, but grows wider constantly. The moon's 
penumbra is also shown in the figure. 

141. Eclipses of the Moon. — When the moon passes 
entirely into the earth's shadow, the eclipse is total. 
When only one side of the moon passes through the 
shadow, the eclipse is partial. Although the almanac 
gives the time when the moon enters the penumbra, yet 
the light shed upon it then is so little diminished that 
it cannot be noticed. There is no perceptible dimming 
of the moon until it almost reaches the shadow. When 
the moon enters the shadow, a notch seems to be cut out 
of the moon. This notch is always round, proving, as 
mentioned in Art. 63, that the shadow must be round, 
and therefore that the earth, which casts it, must 
be a sphere. In a partial eclipse this round notch 
growls larger and larger until the middle of the eclipse, 
then it grows less until it is over. In a total eclipse 
the shadow gradually covers the whole moon. When 
totally eclipsed, the moon can generally still be seen, 
shining with a faint reddish light. This is caused by 
the sun's rays being bent around the earth to the moon 
by the refraction of the earth's atmosphere (Art. 105). 



ECLIPSES. 147 



The light which thus reaches the moon is red, because 
the moisture in the earth's atmosphere absorbs the 
other colors of the sunlight, but allows the red to pass 
through. The sun itself sometimes looks red when 
rising or setting, from the same cause. When only a 
small part of the moon is eclipsed, that part is gener- 
ally entirely invisible to the eye, because the brightness 
of the rest overpowers its feeble light. 

The magnitude of an eclipse is expressed by men- 
tioning the fractional part gf the moon's diameter 
which is covered by the shadow. If the magnitude is 
1, the eclipse is just total ; if more than 1, the moon is 
farther within the shadow. The greatest possible du- 
ration of a total eclipse of the moon is about one and 
three-fourths hours. 

142. Eclipses of the Sun. — When the moon passes be- 
tween the sun and the earth, the sun is eclipsed. 1 At 
all places in the moon's shadow (see Fig. 34) the sun 
is wholly hidden, and the eclipse is total. In the pe- 
numbra the sun is partly hidden, and the eclipse is 
partial. The sun and the moon are apparently about 
the same size ; but, as the distances of both from the 
earth vary somewhat (Arts. 70, 120), their apparent sizes 
vary a little. When the moon is nearest to us, and the 
sun farthest off, the moon will seem larger than the 
sun, and will entirely cover it. The moon's shadow 
then reaches the earth, as shown in the figure, and the 
eclipse is total. But if at the time of the eclipse the 



1 It may not be amiss to remark that the darkness which followed 
the crucifixion of Christ could not have been caused by a solar 
eclipse, for the feast of the Passover, during which the crucifixion 
took place, was always held vXfull moon. 



148 



ASTRONOMY. 




Fig. 34. — Eclipses of the Sun and Moon. 



sun is nearest to us, 
and the moon farthest 
off, the moon would 
seem smaller than the 
sun, and would not 
hide it all. The 
moon's shadow would 
not reach quite to the 
earth. In this case a 
ring of light would 
be seen around the 
edge of the sun. The 
eclipse is not total, 
but annular} 

143. Total Eclipses 
of the Sun. — The sec- 
tion of the moon's 
shadow which falls 
upon the earth is very 
narrow, never as much 
as 200 miles wide. 
And though two or 
\ more solar eclipses 
Of occur every year, yet 
/ a total eclipse of the 
sun is a very rare oc- 
currence at any one 
place. The next one 
visible in the eastern 
United States will be 
in the year 1900. Par- 



1 An / nular, from Latin annulus, a ring. 



ECLIPSES. 



149 




Fig. 35. — A Total Eclipse of the Sun. 



tial eclipses of the sun may last two or three hours, but 
total eclipses rarely exceed five or six minutes. Yet so 
wonderful is the sight, and so great are the opportuni- 

13* 



150 ASTRONOMY. 



ties then afforded of studying the sun, that astronomers 
travel thousands of miles to witness them. A total 
eclipse of the sun is one of the sublimest of phenom- 
ena. The crescent of sunlight becomes narrower and 
narrower, until presently the great shadow is seen 
rushing over the earth towards us with immense rapid- 
ity, and in an instant is upon us. The surface of the 
moon is as black as ink. At various places on its edge 
the red prominences stand out in fantastic shapes, great 
tongues of flame projecting many thousands of miles 
beyond the sun's chromosphere, from which they come. 
Surrounding this is the silvery corona, brilliant at the 
sun's edge, but fading out to imperceptibility. The 
darkness varies according to the duration of the eclipse 
and the clearness of the sky, but it is usually too great 
to allow ordinary print to be read. The brighter stars 
are visible. Animals seem to think that night has come. 
No wonder that uncivilized peoples have always feared 
eclipses of the sun. We cannot behold them without 
awe. Suddenly the light bursts forth, the great shadow 
flies away as fast as it came, and the most wonderful 
spectacle of the generation is over. 

144. Number of Eclipses. — At every eclipse season 
there is certain to be one eclipse of the sun, and there 
may be two. Besides, the moving of the eclipse season 
backward nineteen days a year may throw a part of a 
third eclipse season into the year. The least number of 
solar eclipses that can occur in a year is two, and the 
greatest number is five. 

The moon can be eclipsed only once at each eclipse 
season, and may not be eclipsed at all. The num- 
ber of lunar eclipses in a year varies from none to 
three. The greatest possible number of eclipses in a 



ECLIPSES. 151 



year is seven, of which four or five are of the sun, and 
two or three of the moon. The least possible number 
is two, both of the sun. We may see from Fig. 34 why 
there are more eclipses of the sun than of the moon. 
The sun is eclipsed whenever the moon is partly or 
wholly between G and H, but, because the shadow 
tapers, the distance HG is greater than the width of 
the shadow at B ; the moon will therefore oftener pass 
between the earth and the sun than it will pass through 
the earth's shadow. 

145. Why ive see more Eclipses of the Moon than of the 
San. — Although eclipses of the sun are about one and 
one-half times as numerous as those of the moon, yet 
at any one place on the earth lunar eclipses are more 
frequently seen. This is because the moon's shadow in 
a solar eclipse does not cover the whole earth, and the 
eclipse is seen only at those places which happen to be 
in the path of the shadow or penumbra. But in a 
lunar eclipse the moon's light is put out, and the eclipse 
is seen over all that half of the earth which is then 
turned towards the moon. 

146. Calculation and Prediction of Eclipses — The Saros. 
— When and where eclipses will be seen can be calcu- 
lated beforehand with great accuracy; but considerable 
knowledge of mathematics is required for this. They 
are always announced in our almanacs, and are fre- 
quently worked out many years in advance. 

Long before eclipses could be calculated, men found 
by observation that they repeated themselves so exactly 
about every eighteen years 1 that they could by this 

1 More exactly, eighteen years and ten or eleven days, according as 
four or five leap-years are included. 



152 ASTRONOMY. 



means be predicted. This is because at the end of this 
time the sun, moon, and earth are in almost precisely 
the same positions as at the beginning, so that they 
again describe the same paths, and, with only an occa- 
sional exception, cause the same eclipses, as in the past 
eighteen years. This round of eclipses was called by 
the ancients the Saws. The discovery of the Saros is 
credited to the Chaldeans, 1 who predicted eclipses by 
it hundreds of years before Christ. 

147. Eclipses as seen from the Moon. — When we have 
an eclipse of the sun, an observer on the moon would 
see only a small, round, ill-defined shadow crossing the 
earth. It would be a partial eclipse of the earth (Fig. 
34). The earth could never be totally eclipsed. 

When we have a total eclipse of the moon, the sight 
from the moon would be a very strange one. The sun 
would be wholly behind the earth, and hence totally 
eclipsed, but his light would be so refracted by the 
earth's atmosphere that a dull red ring of light would 
surround the black earth. 

1 Eclipses, especially total eclipses of the sun, were greatly dreaded 
by the ancients, and are still dreaded by uncivilized peoples. The 
Hindoos believe that in a solar eclipse some monster is -trying to 
swallow the sun. At these times they all turn out with gongs and 
every possible noise-producing instrument, and keep up the loudest 
and most hideous noises until the frightened monster disgorges his 
fiery mouthful. 

The foreknowledge of an eclipse of the moon was once of great 
service to Columbus, when, in 1504, he was wrecked off the coast of 
Jamaica. When neither threats nor persuasion would induce the 
natives to furnish him with food, he told them that their Great Spirit 
was displeased with them for their treatment of him, and that the 
moon would that night be darkened. When the eclipse came, the 
Indians were convinced that Columbus had told the truth, and 
hastened to bring supplies for himself and his crew, beseeching him 
to pray that the Great Spirit might receive them again into his favor. 



THE SUPERIOR PLANETS. 153 



CHAPTER VII. 

THE SUPERIOR PLANETS. 
MAKS. cf 

Distance from the Sun, 142,000,000 Miles. Diameter, 4200 
Miles. Length of Year, 2 Years. 1 Length of Day, 24£ Hours. 
Specific Gravity, 4. Two Satellites. 

148. Relations to the Solar System. — Next outside of 
the earth's orbit comes Mars, 2 the last of the group of 
four smaller planets to which the earth belongs. Ex- 
cept Venus, Mars is the nearest of all the planets to 
us ; and next to Mercury, it is the smallest of the prin- 
cipal planets. Its surface is but little more than one- 
fourth, and its mass is about one-ninth, of those of the 
earth. One would there receive less than one-half the 
light and heat that he receives upon the earth. 

149. Motions and Phases. — Unlike Mercury and 
Venus, Mars in its revolution around the sun passes 
entirely around the earth. Its motion among the stars 
is generally direct, or eastward, but when near opposi- 
tion, as explained in Art. 24, it seems to us to move 
westward. Being an outside planet, Mars is never seen 
as a crescent, but when 90° from the sun the telescope 

1 These tabular statements are given in round numbers for con- 
venience in remembering them. For the exact data, see Art. 27. 

2 Mars was the god of war. The symbol of the planet (<?) repre- 
sents his shield and spear. 



154 ASTRONOMY. 



shows it to be decidedly gibbous. It is then of about 
the same shape as the moon three days before or after 
being full. 1 At opposition Mars is nearest to the earth, 
and of course brightest. The oppositions occur every 
twenty-five or twenty-six months. The average dis- 
tance of Mars from the earth at opposition is less than 
fifty millions of miles, but if the opposition should 
occur when Mars is nearest to the sun and the earth is 
farthest from the sun, their distance apart would be 
only thirty-five millions of miles. 

150. Description of Mars. — To the naked eye the 
most noticeable feature about Mars is his fiery-red 
color: he is the reddest of all the heavenly bodies. 
Although he does not come so near to us as Venus, 
yet Mars is in a better position for observation than 
any of the rest of the planets except our own, because 
at opposition, when he is nearest to us, the whole of 
his bright surface is turned towards us, while when 
Venus is nearest to us her dark side is towards us. 
The moon alone, of all the heavenly bodies, is bettei 
situated in this respect. Indeed, we are not certain 
that the real surface of any of the rest of the planets 
has ever been seen. In a large telescope patches of dif- 
ferent colors are seen upon the surface of Mars which 
bear a strong resemblance to land and water. The 
parts supposed to be seas have a greenish hue, like our 
seas, while the parts which have been taken to be land 
are red, which has been ascribed to the color of the 
soil and rocks, perhaps like our red sandstone. The 
red parts overpower the green, and give their color to 
the planet. Maps and globes of Mars have been made, 

1 Can there be a transit of Mars ? 



THE SUPERIOR PLANETS. 



155 



upon which these supposed continents and seas are 
drawn and named, the names given them being those 
of various famous astronomers. Unlike the earth, 
Mars seems to have more land than water, and the 
seas there are long and narrow. But the most striking 
features of Mars's surface are two brilliant white spots 
near his poles. They are probably ice and snow, such 
as are found about the poles of the earth. And they 
seem to decrease when in summer they are turned 
towards the sun, and to increase again when turned 
from the sun in 
winter, just as the 
ice and snow 
about the earth's 
poles do. Be- 
sides these resem- 
blances to the 
earth, Mars has 
seasons like ours. 
His equator makes 
a somewhat larger 
angle with his or- 
bit than the angle 
between our equa- 
tor and the eclip- 
tic. And his orbit 

is more eccentric than the earth's, so that the difter- 
ence between his summer and winter distances from the 
sun is greater. These two causes make the changes 
of seasons, then, greater than upon the earth. The 
markings upon the surface of Mars have enabled us to 
determine the time of his rotation with great exactness. 
It is a little more than twenty-four and one-half hours. 




Fig. 36.— A Telescopic View of Mars. 



156 ASTRONOMY. 



151. The Satellites of Mars. — Before 1877, Mars was 
not known to have any satellites. At the opposition 
which occurred that year the planet came very near the 
earth, and gave an unusually good opportunity of ob- 
serving him. At that time Prof. Hall 1 began to search 
carefully for satellites of Mars with the great twenty- 
six-inch telescope in the Naval Observatory at Wash- 
ington. In August of that year he found two satellites. 
These satellites are very small and very close to the 
planet. They are too small to be measured, but, esti- 
mating their size from the amount of light they reflect, 
Prof. Pickering 2 concludes that the diameter of the 
inner is about seven, and of the outer one about six, miles. 
The outer one is about 12,000 miles from the surface of 
Mars, while the inner one is not quite 4000. But the 
most remarkable fact about the satellites is that the 
inner one revolves about Mars in about seven and one- 
half hours, less than one-third of the time that it takes Mars 
to turn on his axis. This causes the inner satellite to rise 
in the west and set in the east. 3 To an observer on Mars 
it would seem to revolve around his planet from west 
to east twice every day, 4 all the while going through its 
changes from new to full and back to new again every 

1 Prof. Asaph Hall, attached to the United States navy, and in 
charge of the great twenty-six-inch telescope at Washington. 

2 Prof. E. C. Pickering, Director of the Harvard College Observa- 
tory, Cambridge, Massachusetts. 

3 Since the inner satellite revolves around Mars from west to east 
faster than the planet itself turns in that direction, it must rise in 
the west and set in the east. 

4 Since the planet turns once in its daily motion while the satellite 
revolves about it three times, and both in the same direction, the 
satellite would seem to an observer on Mars to revolve about his 
planet only twice a day. 



THE SUPERIOR PLANETS. 157 

seven or eight hours. As the outer moon rises in the 
east and sets in the west, like ours, the two moons meet 
each other in the sky. The discovery of the moons of 
Mars is justly regarded as one of the greatest in recent 
astronomy. 

152. Observation of Mars.— Although so interesting 
an object to the possessor of a large telescope, for the 
owners of small telescopes Mars has little interest. 
Small instruments show none of the markings on the 
planet distinctly, if at all ; and he has very little change 
of phase such as makes Mercury and Venus interesting. 
As has been said, Mars is much the brightest at oppo- 
sition, when his brilliancy and redness make him an 
interesting object to the naked eye; nor will a small 
telescope add much, if any, interest to him. Like all 
heavenly bodies when in opposition, Mars will then 
rise about the time the sun sets, and shine all night. 
This fact, together with his fiery redness, will make it 
easy to distinguish him. Every fifteen or seventeen 
years Mars comes especially near to the earth at oppo- 
sition. His brilliancy then almost rivals that of Venus 
and Jupiter. These near approaches occur in 1892 and 
1909. The satellites of Mars can be seen only in first- 
class telescopes, 1 and then for but a few months about 



1 The foolish statement that Mars ; s moons can be seen by looking 
at the planet in a common looking-glass is sometimes heard, and even 
seen in the newspapers. The points of light which are seen in the 
looking-glass beside the image of Mars are faint reflections of the 
planet's light from the inner and outer surfaces of the glass, while the 
main reflection is from the quicksilver behind the glass. Any of the 
bright stars, and even the moon, will show such " moons "ina look- 
ing-glass. But it a piece of polished metal be used as a mirror, they 
will disappear from Mars as well as from the rest. 

14 



158 



ASTRONOMY. 



the time of the planet's opposition. No telescope in 
the world will show them when Mars is in the farther 
part of his orbit. 

Schiaparelli of Milan thinks he has discovered some 
curious markings on Mars which he calls canals. These 
always end in a sea or another canal, and are perfectly 




Fig. 36.— Map of Mars. 

straight. They are sometimes fifty miles wide and one 
thousand long, and their exact nature is a matter of 
conjecture. Sometimes they seem to be double, a 
second line parallel to the first being noticeable. 
They make a network over the continents, and some- 
times seem to disappear. The map is a copy of 
Schiaparelli's. 



THE MINOR PLANETS. 159 



THE MINOR PLANETS. 

Distance from the Sun, 200,000,000 to 325,000,000 Miles. 
Diameters, from about 300 Miles down. Lengths of Years, 
3 to 7 Years. Lengths of Days and Specific Gravities un- 
known. Number discovered over 300. 

153. Relations to the Solar System. — Between the inner 
group of four small planets and the outer group of 
four large ones is a wide gap, in which hundreds of 
very small planets are revolving about the sun. These 
are called Planetoids, or, better, Minor Planets} Their 
number is unknown, and may be very great; but the 
mass of all of them put together is much less than 
that of the smallest of the principal planets. The 
orbits of these planets are much more elliptical than 
those of the larger ones ; some of them are twice as 
far from the sun at aphelion as at perihelion. And 
while the orbits of the principal planets all make small 
angles with the ecliptic, the orbits of some of the 
minor planets make large angles with it. Their dis- 
tances from the sun vary greatly. Some of them 
almost intersect the orbit of Mars, others swing out 
nearly as far as Jupiter's orbit, while the whole space 
between is filled with them. But their orbits are so 
entangled that, if they were actual rings, scarcely one 
of them could be picked up without disturbing all the 
rest. 

154. The Discovery of the Minor Planets. — The gap 
between Mars and the outer group of planets was long 



1 Also often called asteroids (star-like) but the term is gradually 
giving way to the more appropriate one of minor planets. 



160 ASTRONOMY. 



since noticed by astronomers. Besides, a law, known 
as Bode's law, 1 had been devised, which, if this gap 
were only filled by a planet in its proper place, ex- 
pressed quite accurately the relative distances of the 
known planets from the sun. These facts led astrono- 
mers, about the beginning of this century, to make 
careful search for the missing planet. Twenty-four 
astronomers divided the zodiac 2 into as many parts, 
and began the search. But on the first day of the 
present century, 3 before they got fairly started, an Ital- 
ian astronomer, an outsider, discovered a small planet 
in the vacant space. In seven years three more were 
found, but after that no- more until 1845. Since that 
time a large number have been found, and sometimes 
ten or twelve are discovered in a year. The number 
already known is over three hundred. 

1 If the series 0, 3, 6, 12, 24, 48, 96, etc., be formed by doubling 
each term after the first to get the next one, and then 4 be added to 
each term, we shall have 4, 7, 10, 16, 28, 52, 100, etc. If we multi- 
ply each of the first four of these numbers by 9,000,000, we shall have 
pretty nearly the distances of the inner group of planets from the 
sun. Fifty-two multiplied by the same number gives about the 
distance of Jupiter, the first of the outside group, while 28 times 
9,000,000 was supposed to be the distance of the undiscovered planet. 
This is called Bode's law, after a celebrated German astronomer who 
died in 1826, but it was devised long before his time by Titius. The 
law held good for all the principal planets until the discovery of 
Neptune, the outermost of the outside group, in 1846, for which it 
was found to fail. These coincidences are now believed to be merely 
accidental. 

2 It will be remembered that the zodiac is a belt of the sky ex- 
tending about 8° on each side of the ecliptic, in which all the prin- 
cipal planets are always found. It was therefore very natural that 
they should examine the zodiac for the new planet. But the minor 
planets, as we now know, are not all within the zodiac. 

* What day was this ? See page 14, note 3. 



THE MINOR PLANETS. 161 

The discovery of these planets is a difficult task. 
None that are now discovered can be seen with the 
naked eye, and in the best telescopes they look exactly 
like very small stars, from which they can be distin- 
guished only by their motions among the stars. Among 
the astronomers who have occupied themselves largely 
with this work, two Americans, Profs. Peters 1 and 
Watson, 2 and Palisa, an Austrian, have been most 
successful. The new planets whose discovery we see 
occasionally announced in the newspapers are always 
some of these. 

The first minor planets found were named after 
various goddesses, and this custom is still adhered to. 
But their great number has made it very difficult to 
find enough such names for them, so that many of 
those lately discovered have not yet been named. 
They are usually designated simply by numbers, w T hich 
show the order of their discovery, the numbers being 
enclosed in circles, thus :©,©,©. 

155. Description of the Minor Planets. — As has been 
said, these planets are very small. Two or three of 
them may rarely and under the most favorable circum- 
stances be seen by the naked eye, but only as very faint 
stars. Many of them can be seen only through good 
telescopes. None of them have any apparent size even 
in the largest telescopes, so that their size can be esti- 
mated only from the amount of light they give. The 
largest of them may be three hundred miles in diam- 
eter, while the smallest yet discovered is probably not 



1 C. H. F. Peters, 1813-1890, Professor of Astronomy at Hamilton 
College, Clinton, New York. 

2 See page 65, note 3. 

I 14* 



162 ASTRONOMY. 



more than fifteen miles in diameter. On these planets 
the attraction of gravity is slight, and one would there- 
fore weigh much less than upon the earth. Sir John 
Herschel remarks that " a man placed upon one of 
the minor planets would spring with ease sixty feet, 
and sustain in his descent no greater shock than he 
does on the earth from leaping a yard." Of their rota- 
tion, surface, atmosphere, etc., we know nothing. Their 
appearance presents no features of interest in any tele- 
scope. 

156. Are the Minor Planets Fragments of one Large 
Planet? — When the first two or three of these planets 
had been found, it was suggested that they might be 
fragments of a larger planet which had from some 
cause burst to pieces. The theory at first seemed 
probable, but it has been long since rejected by as- 
tronomers. So far as we can tell, they have been re- 
volving about the sun ever since the solar system was 
created. 1 



1 And yet the facts of their revolving about the sun all in a ring- 
together, and of having their orbits so entwined, seem to indicate 
some connection among them originally. Perhaps we might regard 
them as a group of great meteoroids revolving about the sun, just as 
many groups of small meteoroids (shooting-stars) are now known to 
be revolving about the sun (Art. 197), and just as Saturn's rings are 
probably dense groups of small meteoroids revolving about him (Art 
169). 



JUPITER. 163 



JUPITEK. % 

Distance from the Sun, 480,000,000 Miles. Diameter, 86,500 
Miles. Length of Year, 12 Years. Length of Day, 10 Hours. 
Specific Gravity, lh Four Satellites. 

157. Jupiter's Relations to the Solar System. — The first 
of the outside group of four planets is Jupiter. 1 He is 
the largest of all the planets, his volume 2 being one 
and a half and his mass 2 two and a half times as great 
as that of all the other planets put together. Com- 
pared with Jupiter, our earth is insignificant. It would 
take about 1400 earths to make a planet as large as 
Jupiter, although 300 earths would make one as heavy 
as he, for Jupiter's specific gravity is less than one- 
fourth of the earth's. Yet his volume and mass are 
only 10 1 00 of those of the sun. At his great distance, 
one would receive from the sun only about -^ as much 
heat and light as upon the earth. If he depends solely 
upon the sun for heat, the temperature of his surface 
must be nearly 500° below zero. Notwithstanding his 
great size, Jupiter's day is not half as long as ours. 
His year is as long as twelve of ours, but, as his equator 
makes an angle of only 3° with his orbit/ he has^no 
changes of seasons of any consequence. 

1 Ju'pi-ter, the chief of the gods. His symbol {%) is perhaps a 
rude representation of an eagle, the bird of Jupiter. 

2 The student must be careful to have a very clear notion of what 
these mean. Volume is size or bulk ; it varies according to the cube 
of the diameter. Mass is quantity of matter ; it varies according to 
the weight. Then if all of the planets could be weighed at the same 
place (at the sun, for instance), Jupiter would weigh two and a 
half times as much as all of the rest together. 

8 How many degrees wide is Jupiter's torrid zone ? his frigid and 
temperate zones ? 



164 ASTRONOMY. 



158. Jupiter's Motions and Phases. — Jupiter is so far 
off that he shows no phases to the ordinary observer. 
His disk in the telescope is always full. But the great 



Fig. 37. — Jupiter, as seen in the Telescope of Haverford College Obser- 
vatory (inverted), January 7, 1882; showing the Shadow of a Satellite on 
its Disk, another Satellite just emerging from in front, and the Oval 
" Red Spot." 

velocity of his rotation upon his axis makes him no- 
ticeably flattened at the poles. Like Mars and all the 
other superior planets, Jupiter has a retrograde motion 
among the stars when he is at opposition. This 
occurs once in about thirteen months. 



JUPITER. 165 



159. Jupiter's Belts. — When seen through a telescope, 
the most conspicuous feature of Jupiter is his belts. 
These are dark bands or streaks stretching across the 
planet. Usually there are two conspicuous ones, just 
above and below the planet's equator. But others are 
often seen, sometimes covering the greater part of his 
surface. They are well shown in Fig. 37. Sir Wil- 
liam Herschel 1 thought that the belts were openings in 
the planet's atmosphere, through which we can see the 
darker surface of the planet itself. Jupiter is certainly 
surrounded by an atmosphere, but so dense and deep 
is it that it is probable that we never see the planet's 
surface. The belts seem to be fissures in the upper 
clouds and atmosphere, through which, we see the 
lower strata of atmosphere and clouds, which are 
darker because they reflect less sunlight. In small 
telescopes the belts are simply dark, but in better in- 
struments they often display colors. This color is usu- 
ally a dull red, but occasionally varied and more bril- 
liant ones have been seen. The cause of these colors 
and their changes is not known. 

160. Spots on Jupiter. — Besides the ever-present belts, 
spots are also sometimes seen upon Jupiter's surface. 
They are commonly dark or of a dull-red color, but 
bright white ones have been seen. The dark ones may 
be openings in the upper atmosphere, while the white 
ones look somewhat like the round masses of white 
cloud which are so common here in summer. Some of 
these spots have remained visible for a considerable 
time, and it is by observations of these that the period 

1 Born 1738, died 1822. A German by birth, but spent all of his 
manhood in England. Generally conceded to be the greatest prac- 
tical astronomer that has yet lived. 



166 ASTRONOMY, 



of the planet's rotation has been determined. The 
various observations show that these spots have some 
motion of their own, so that it is not possible to deter- 
mine the period of Jupiter's rotation as exactly as that 
of Mars. A very large spot, known as " the red spot," 
which appeared in 1878, remained for several .years about 
the same shape and size. Then it gradually faded away, 
and again reappeared. It is in the southern hemisphere 
of the planet, rather oval in shape, about 24,000 miles 
long, and of a dull-red color. It appears to be quite 
permanent, though showing changes of outline, color, 
and intensity. 1 

161. Jupiter's Temperature. — There is strong evidence 
that Jupiter is* still very hot. The sun's rays are too 
feeble there to raise the thick clouds which constantly 
envelop him, and the great changes continually going 
on in his atmosphere must be caused by intense heat 
within. The earth shows plainly that it was once 
in a molten state, and if it was created at the same 
time as Jupiter, the great mass of the latter would 
keep him hot long after the earth had cooled off. If 
the body of the planet has yet solidified, it is still prob- 
ably white-hot. So that Jupiter is more like the sun 
than like the earth. And indeed there is evidence 
that he actually gives out some light of his own, even 
through his dense cloudy atmosphere. The amount of 
this, however, if any, cannot be great : the most of his 
light is reflected sunlight. 

162. Jupiter's Satellites. — Among the first objects 
which Galileo's little telescope revealed to him were 



1 As determined by observations on this spot, Jupiter rotates in 
about 9 h. 55 m. 35 sec. 



JUPITER. 



167 



four moons revolving around Jupiter. They are almost 
bright enough to be seen by the naked eye, and indeed 
they have on rare occasions been thus seen by persons 




Fio. 38.— Jupiter and his Satellites. 



having exceptionally good eyesight. Were it not for 
the overpowering brightness of Jupiter, they would 
be usually visible. The satellites are known by their 
numbers, the one nearest the planet being called the 
first? the next one the second? and so on. The near- 
est one is a little farther from Jupiter than the moon 
is from the earth, the fourth is more than a million 
miles off. The second is the smallest, and is about 
the size of our moon ; the third, which is the largest, 
is about 3700 miles in diameter, being considerably 
larger than Mercury. They all revolve about Jupiter 
in the same direction that the planets revolve about 
the sun, from west to east. The first makes his revo- 
lution in If days, the fourth in less than 17 days. 
Their orbits are all nearly circular, and lie almost in 
the plane of Jupiter's orbit. On this account they 



1 They are generally designated by the Koman numerals, I., II.; 
III., IV. 



168 ASTRONOMY. 



are never far out of a straight line passing through 
Jupiter (Fig. 38). 

163. Eclipses and other Phenomena of Jupiter's Satellites. 
— Jupiter's moons, like our own, are eclipsed, but, 
owing to the size of the planet's shadow and the coin- 
cidence of their orbits with the plane of Jupiter's orbit, 
they are eclipsed much oftener than our moon. The 
three inner satellites are eclipsed at every revolution, 
and the outer one generally is. In Fig. 39 the satellite 





Fig. 39. — Phenomena of Jupiter's Satellites. 

is eclipsed while passing from 1 to 2. It is then en- 
tirely invisible, which proves that if Jupiter emits any 
light of his own it must be very little. From 3 to 4 
the satellite is again invisible, because it is behind the 
planet. This is an occultation of the satellite. 1 and 3 
are the points of disappearance, 2 and 4 of reappearance. 
The orbit of the first satellite in the figure shows that 
the occultation may begin before the eclipse ends. On 
the opposite side of its orbit, from 5 to 6, the satellite 
is between the sun and Jupiter, and partly eclipses the 
planet. The shadow of the . satellite, a small round 
black spot, is seen crossing Jupiter's disk. From 7 to 
8 the satellite transits across the planet, and in a good 



JUPITER. 169 



telescope may be seen, being sometimes a little brighter 
and at other times a little darker than the surface of 
the planet behind it. Sometimes the satellite and its 
shadow will be seen upon Jupiter at the same time, as 
the orbit of the first satellite in the figure shows. The 
passing of the satellite or shadow on the edge of the 
planet is called ingress ; the passing off, egress. 

It was by observations of Jupiter's satellites that the 
velocity of light was first found. See Art. 239. 

164. Observations of Jupiter. — Next to Venus, Jupiter 
is the brightest of the planets, and of course far brighter 
than any of the fixed stars. But while Venus, being 
an inferior planet, is never seen far from the sun, and 
never at a late hour of the night, Jupiter, being a su- 
perior planet, may be at any distance from the sun, 
and may shine all night. These facts, together with 
the information given about the times when it is an 
evening and when a morning star, will usually enable 
one to recognize Jupiter. And when once found, one 
may keep track of him from year to year, for, on ac- 
count of the great length of his orbit, he moves but 
slowly among the stars. As he takes twelve years to 
journey around the sun, he moves through one con- 
stellation in the zodiac each year. Although Jupiter 
is always bright, and can always be observed with 
interest, except when too near the sun, yet he is of 
course brightest and best seen when in opposition. 
With a telescope which magnifies sixty times its di- 
ameter will seem as large as that of the full moon. 
With larger and more powerful glasses it becomes 
an object of great interest, and the surface of the 
planet, as it slowly revolves, may be studied night 
after night with profit. Through even a very 

15 



170 ASTRONOMY. 



small telescope Jupiter is interesting, and his moons 
may be seen through a small spy-glass or a common 
opera-glass. They look like small stars, and will be 
easily distinguished by their being about in a straight 
line with the planet. Generally one or two will be 
on one side, and the others on the other ; rarely all 
four will be seen on one side, and sometimes one or 
two may be invisible from being eclipsed or in occul- 
tation. Under favorable circumstances (see p. 290), a 
two-inch telescope will show the principal belts of Jupi- 
ter, and in larger instruments the other belts and spots 
may be seen. Great activity is often manifest upon 
Jupiter, and all who can do so should watch it, ob- 
serving the period of rotation from the spots, noting 
the colors and any changes or peculiarities, and mak- 
ing maps of its surface. When a spot can be ob- 
served on Jupiter, those having suitable instruments 
ought to note the times of the passage of the spot over 
the planet's central meridian for as long a time as pos- 
sible, to determine the period of the planet's rotation 
In all astronomical observations the time must be care- 
fully noted. 

165. Observations of Jupitefs Satellites. — For telescopes 
of moderate size the most interesting observations of 
Jupiter are those upon the phenomena of his satel- 
lites. All of the phenomena described in Art. 163 
are predicted in the Nautical Almanac, 1 and may be 

1 The Nautical Almanac is published -by the United States gov-, 
ernment at Washington, D.C., for. each year, an<| comes out about 
three years in advance* , It is indispensable to the navigator and the 
astronomer. It may be obtained by sending one dollar, to the Bureau 
of Navigation, Washington, D.C. 

The following data are taken from page 460 of the Almanac for 



JUPITER. 



171 



observed at the times given there. 1 The Nautical 
Almanac also gives figures showing the positions of 
the satellites for every day in the year, by which the 
different satellites can be distinguished. For the ap- 
parent order of the distances of the satellites from the 
planet may not be the real order of their distances. If 
Satellite IV. were almost in front of or behind Jupiter, 
it might appear to be the nearest of all. Satellite III. 
may generally be distinguished by its size. 

When a transit or occultation is to be observed, every- 
thing should be ready at least five minutes before the 
predicted time ; and the error of the watch which is 
to be used for taking the time must be known. In an 
eclipse, the times of the first diminution of brightness 



1883. They give the time of the phenomena about the period of 
Jupiter's opposition in 1883. 

DECEMBER, 1883. 



Date/ 


No. of 
Satellite. 


Phenomenon. 


Phase. 


d. 


h. m. s. 








21 


10 45 


I. 


Shadow. 


Ingress. 




11 26 


I. 


Transit. 


Ingress. 




13 5 


I. 


Shadow. 


Egress. 




13 43 


III. 


Occultation. 


Reappearance. 




13 46 


I. 


Transit. 


Egress. 


22 


7 57 6.1 


I. 


Eclipse. 


Disappearance. 




10 55 


I. 


Occultation. 


Reappearance. 




14 12 30.4 


II. 


Eclipse. 


Disappearance. 




18 26 


n. 


Occultation. 


Reappearance. 


! 23 


7 34 


i. 


Shadow. 


Egress. 




8 13 


i. 


Transit. 


Egress. 


24 


8 25 


ii. 


Shadow. 


Ingress. 




9 41 


ii. 


Transit. 


Ingress. 




11 19 


ii. 


Shadow. 


Egress. 



(" Shadow," in the third column, means the transit of the satellite's 
shadow across Jupiter's disk.) 

1 The dates in the Almanac are in Washington astronomical time. 
These must he reduced to civil time (see Art. 89), and then the cor- 
responding local time found, as in Art. 102. The dates given may be 
one or two minutes in error. 



172 ASTRONOMY. 



and of final disappearance should be noted. At reap- 
pearance the order will be reversed, the times of first 
reappearance and of complete restoration of brightness 
being noted. Two persons can take time better than 
one : while one observes, the other can consult the watch. 
In occultations the time of first contact, that is, when the 
satellite first touches the edge of the planet, and that of 
last contact, when the last part of the satellite disappears 
behind the planet, should be noted if the telescope is 
large enough to show them. The transits of the satel- 
lites and the shadows are more interesting, but require 
rather better telescopes. To see them well, a four-inch 
glass is needed. The transit of the shadow is seen more 
easily than that of the satellite, although the times of 
ingress and egress are harder to determine than those 
of the satellite. Sometimes a satellite and its shadow 
may be seen upon the planet at the same time. The 
first and last contacts in a satellite's transit, like those 
of the occupation, should be noted. If the state of the 
atmosphere is such that the exact times cannot be deter- 
mined, they may be carefully estimated. Records of 
all observations, with notes upon the state of the atmos- 
phere and the reliability of the observations, should be 
kept. 1 

1 Fig. 39 shows that when the earth is at E the eclipses come before 
the occultations, and the transits of the shadows before the transits of 
the satellites. When the earth is in this part of its orhit, Jupiter 
rises more than twelve hours after the sun, and therefore in general 
rises in the night. But when the earth is at F in the figure, the oc- 
cultations and transits of satellites come first. And when the earth 
is in this part of its orbit, Jupiter rises less than twelve hours after 
the sun, or in general in the daytime. The following approximately- 
correct rule may be derived from these facts : If Jupiter rises in the 
night, the eclipses will precede the occultations, and the shadows will 



SATURN. 173 



In ordinary telescopes the satellites will be simply 
points of light like the stars. In large instruments 
their disks can be seen, and some curious and unex- 
plained changes in their appearance have been ob- 
served. 

Observations of the eclipses of Jupiter's satellites are 
sometimes used to determine the diflerence of longitude 
between two places (Art. 102) ; but the gradual disap- 
pearance of the satellites as they pass into the shadow 
makes the determination of the exact time of the eclipse 
so uncertain that this method is not very often used. 

SATUKN. h 

Distance from the Sun, 890,000,000 Miles. Diameter, 73,000 
Miles. Length of Year, 30 Years. Length of Day, 10 Hours. 
Specific Gravity, f . Eight SateUites. 

166. Relations to the Solar System.— Next beyond 
Jupiter, but almost twice as far from the sun, is Saturn. 1 
Next to Jupiter he is the largest of the planets. His 
volume is 700 times that of the earth. But his mass 
is only about 100 times as great, which shows that 
Saturn must be made of very light material. His 
specific gravity is the least of all the planets, and he is 
only three-fourths as heavy as a globe of water of his 
size. He would float in water. What has been found 
to be true of the densities of Jupiter and Saturn is also 

cross the planet ahead of the satellites ; but if Jupiter rises in the 
daytime, in which case it will be found up some distance in the sky- 
in the evening, the occultations and transits of the satellites will come 
first. 

1 Saturn, the god of time, father of Jupiter. His symbol (h) repre- 
sents an old-fashioned scythe or sickle. 

15* 



174 



ASTRONOMY. 



true of the other two outside planets. None of them 
differ much from water in density ; all are much lighter 
than the four inner planets. But in all of these we see 




Fig. 40.— Saturn. 



and measure the outside of their atmospheres, so that 
what we give as the specific gravity of the planet is 
really the average specific gravity of the planet and its 
atmosphere taken together. If the atmosphere is deep, 
the real body of the planet is smaller and denser. The 
size and density of the real planet we cannot determine, 
because it cannot be seen through the atmosphere. 
Since Saturn is nearly twice as far from the sun as 
Jupiter, one would there receive only about one-fourth 
as much light and heat from the sun as upon Jupiter. 
167. Description of Saturn. — Saturn's enormous dis- 



SATURN. 175 



tance from us makes it impossible for us to see much of 
his surface. But, so far as we can tell, the body of the 
planet is very much like Jupiter. He has faint belts, 
and probably he has not yet cooled off into a solid 
body like the earth, and is surrounded by a dense at- 
mosphere and clouds. Spots are sometimes seen upon 
his surface, and when seen have been used to determine 
the time of his rotation upon his axis. He is even more 
flattened at the poles than Jupiter. 

168. Saturn's Rings. — The most remarkable feature 
about Saturn, and one possessed by no other body in 
the solar system, is a set of enormous rings surround- 
ing him. Fig. 40 well represents these remarkable 
rings, and also the planet itself. Through a small tele- 
scope one bright ring is seen, but a larger instrument 
shows that this ring is divided into two, one within the 
other. The inner ring is the wider of the two. In 
1850, Prof. Bond 1 discovered a very faint third ring 
inside of the others, a continuation of the inner bright 
ring; it is commonly called the dusky ring. The di- 
ameter of the rings from outside to outside is about 
170,000 miles, and the two bright rings together are 
some 30,000 miles wide. 2 The dusky ring extends 
about half-way from the inner bright ring to the body 
of the planet (shown in Fig. 40). The rings are very 
thin, probably not more than 100 miles through. A 
ring of good writing-paper one foot in diameter will 



1 G. P. Bond (1826-1865), associate of, and successor to, his father, 
W. C. Bond (1789-1859), at Harvard Observatory. 

2 How far is the inner edge of the bright rings from the body of the 
planet ? 

The division between the two bright rings is less than 2000 miles 
wide. 



176 ASTRONOMY. 



represent their proportions pretty correctly. Other di- 
visions of the rings have been suspected by astrono- 
mers, but if any have been seen they must have been 
but temporary ones. 

169. The Constitution of the Rings. — Mathematical 
reasoning 1 has shown that in all probability the rings 
are composed of small, distinct particles of matter, 
too small to be seen separately, and so close together 
that we cannot see through them; just as a column 
of smoke or a cloud looks like one solid mass, when it 
is really made up of a great many little particles of 
matter crowded together. In the dusky ring the par- 
ticles may not be crowded together so thickly as in the 
bright rings. The rings shine by reflecting the sun's 
light. The particles composing the rings must revolve 
about the planet, or they would fall to his surface. 
The rings are then really a cloud of small satellites 
chasing each other around in a ring about Saturn. 
The time of rotation is thought to be a little greater 
than that of the planet. 

170. Appearances of Saturn's Rings. — The rings are 
not perpendicular to the earth's orbit, but inclined to 
it at an angle of 27°. We never, therefore, get a full 
front view of the rings; when widest open, they seem 
to us like rather narrow ellipses. Twice during Sat- 
urn's revolution about the sun, or about every fifteen 
years, the edge of the ring is towards the earth. So 



1 Prof. Benjamin Peirce (1809-1880), of Harvard University, 
proved that the rings could not be continuously solid, and thought 
they were probably liquid. But Prof. J. Clerk Maxwell (1831- 
1879), of Cambridge University, England, proved that they could 
not be liquid, and hence inferred their probable constitution to be as 
given above. 



SATURN. 



177 



thin is the ring that at this time it is entirely invisible 
in ordinary telescopes, and Saturn seems to be simply 
a round planet like the rest. In powerful telescopes 
the ring at this time looks like a fine wire running 
through the centre of the planet. This occurred in 
February, 1878, and will occur again in December, 




Pig. 41.— The Different Appearances of Saturn's Rings. 



1891. After passing this point, the ring gradually 
opens wider and wider, until, when half-way to the 
next disappearance, we see it at an inclination of about 
27°, the most favorable position for its observation. 
As the figure shows, this is its position in 1885 and in 
1899. 

171. Saturn's Satellites. — Saturn has eight satellites, 
— twice as many as any other planet. The nearest is 
about half as far from Saturn as the moon is from the 



178 ASTRONOMY. 



earth, the farthest is more than 2,000,000 of miles 
away. The sixth satellite is the largest, being over 
3000 miles in diameter ; the smallest ones are too small 
for measurement. The sixth can be seen in any tele- 




Fig. 42. — Saturn and his Satellites. 



scope. The eighth is as bright as the sixth when west 
of the planet, but can be seen only in large telescopes 
when east of it. It is supposed that one side of it is 
much darker in color than the other, and as it turns on 
its axis the bright and dark sides are turned towards us 
alternately. From the fact that it always disappears 
upon the same side of Saturn, it is inferred that, like 
our moon, it rotates once on its axis during each revo- 
lution about the planet. The smallest satellites are 
visible only in the largest telescopes. 

The satellites revolve about Saturn nearly in the 



SATURN. 179 



plane of the rings, — at an angle, therefore, of nearly 27° 
to Saturn's orbit as well as to our own ; for Saturn's 
orbit is nearly in the same plane as the earth's. Hence 
they are seldom eclipsed. In passing around the planet 
they generally cross above or below the shadow. When 
they do occur, on account of their great distance and 
the small size of most of the satellites, the eclipses and 
transits are of little interest. 

172. Views from Saturn. — If an observer on Saturn 
could see through its atmosphere, the view of the 
heavens must be striking. Although the sun there 
has but Jjy of the diameter that he has to us, and sheds 
then scarcely jfa of the light and heat that we get, yet 
the eight moons and the wonderful rings would be an 
interesting spectacle. The rings form broad, bright, 
rainbow-shaped arches crossing the heavens, each side 
being bright and dark alternately for fifteen years at 
a time. The rings must cause frequent and long-con- 
tinued eclipses of the sun. 

173. Observation of Saturn. — To the naked eye Saturn 
is not an object of much interest. It is not nearly so 
bright as Venus or Jupiter, and is surpassed in bright- 
ness by one or two of the fixed stars. On account of 
its distance, its brightness does not vary much, although 
it is somewhat increased when the rings are opened 
wide. Hence it is not so easily found as the brighter 
planets ; but the data given on the first page and in the 
body of an almanac, together with the fact that it is a 
strange star in the constellation 1 where it happens to be, 
will, with a little attention, enable one to find it. And 

1 It will be remembered that the planets are not to be looked for 
all over the sky. They are always in the constellations along the 
zodiac. 



180 ASTRONOMY. 



when once found it may be followed from year to year, 
for its motion is so slow that it is two and one-half years 
in passing through a single constellation. 

In a telescope Saturn is unmistakable, and is the most 
beautiful object in the heavens (Fig. 40). The rings, 
with one and sometimes two of the satellites, can be 
seen with a telescope of two or three inches of aper- 
ture. The other satellites and the marks on the planet 
require larger instruments. The rings are widest open 
in 1885 and again in 1899 ; the best views of the planet 
were had a few years before and after these dates. 
The years 1882-1885 were exceptionally favorable. In 
1891-92 the edge of the ring is towards the earth. 
This is of great interest to astronomers who have large 
telescopes, but for owners of small telescopes Saturn 
about this time possesses little interest. 



URANUS. 181 



UKANUS. 6 

Distance from the Sun, 1,800,000,000 Miles. Diameter, 32,000 
Miles. Length of Year, 84 Years. Length of Day Unknown. 
Specific Gravity, 1£. Four Satellites. 

174. Position and Description of Uranus. — Uranus 1 is 
the third in order from the sun, and the smallest in 
size, of the outer group of planets. In the largest tele- 
scopes no markings have ever been certainly seen upon 
it : it shows only a bright, round disk. From its 
great size and its position, it is likely that it resembles 
Jupiter, and, like him, may not have cooled off yet. 
But of this we have no direct evidence. In a large 
telescope Uranus has a greenish tinge. 

175. Discovery of Uranus. — This planet was discov- 
ered by Sir William Herschel in 1781. Herschel was 
a German music-teacher, who had settled in England 
and was at this time a church organist in the city of 
Bath. Having a great fondness for astronomy, he 
made a number of telescopes with his own hands, and 
became a diligent amateur observer. At the time of 
his discovery of Uranus he was almost entirely un- 
known as an astronomer. But this discovery at once 
made him famous. George III. made him his private 
astronomer, and gave him a pension of one thousand 
dollars a year. The rest of his life was devoted en- 
tirely to astronomy, in which he made many great dis- 
coveries. 

When first discovered, Uranus was supposed to be a 



1 U'ranus, the oldest of the gods. So named because this was sup- 
posed to be the most distant of the planets. 

16 



182 ASTRONOMY. 



tailless comet ; but it was soon found to be a planet. 
Its discovery awakened the greatest enthusiasm in the 
scientific world. Not even a new satellite had been 
discovered for nearly one hundred years, and all of the 
known planets had been known from the very earliest 
times. It was found that Uranus had frequently been 
seen before, but had always been taken for a fixed 
star. 

176. Satellites of Uranus. — Sir William Herschel an- 
nounced the discovery of six satellites to this planet, 
but it is now generally agreed that he really saw only 
two, and that he was mistaken about the other four. 
Two others were afterwards discovered, and it is cer- 
tain that these four are all that have ever been seen, 
although some of the older works on astronomy still 
credit Uranus with six. The last two discovered are 
extremely faint; very few telescopes will show them 
at all. 

The most remarkable fact about these satellites is, 
that, unlike every body in the solar system that we 
have yet considered, they do not revolve in their orbits 
from west to east. They revolve around Uranus almost 
from north to south, 1 and what little motion they have 
in the other direction is towards the west, and therefore 
retrograde. 

177. Observations of Uranus. — When nearest to the 
earth, Uranus is just visible to the naked eye, and looks 
exactly like a very faint star. Unless, therefore, one 
knows just where it is, Uranus cannot be distinguished 
by the naked eye from the stars. Its position is best 

1 That is, in the direction which is to us north and south. It may- 
be that Uranus rotates in the same direction, and that his moons there- 
fore revolve about him nearly parallel to his equator. 



NEPTUNE. 183 



found from the Nautical Almanac, where its right 
ascension and declination (Art. 31) are given for each 
day in the year. These will show its position on a 
good celestial globe or map, from which it may be 
found. 1 But it is scarcely worth finding, for to the 
naked eye, or in an ordinary telescope, it possesses 
no great interest. 

NEPTUNE, tp 

Distance from the Sun, 2,800,000,000 Miles. Diameter, 35,000 
Miles. Length of Year, 165 Years. Length of Day Unknown 
Specific Gravity, 1£. One Satellite. 

178. Position and Description. — Neptune 2 is the outer- 
most planet of the solar system. It is rather larger 
than Uranus, and it is not unlikely that it resembles 
him and the others of the group of great planets. Of 
its condition nothing can be determined with any cer- 
tainty. The largest telescopes show nothing but a 
small bright disk. No markings can be seen upon 
it, and therefore the period of its rotation cannot be 
determined. 

179. Discovery of Neptune. — After Uranus had been 
discovered and watched* for a number of years, it was 
found that its motion was not quite what it should 
be if acted upon solely by the attraction of the sun 
and the known planets. It is true that this deviation 
was very slight: it amounted altogether only to 2', 

1 When a telescope is properly mounted, it may be pointed at once 
to any right ascension and declination, and a planet or star, even if 
invisible to the naked eye, can thus be found. 

2 Nep'tune, god of the sea, son of Saturn, and brother of Jupiter. 
The planet's sign is the trident of the god. 



184 ASTRONOMY. 



or one-sixteenth of the apparent diameter of the moon. 
If one star were where astronomers had calculated 
that Uranus ought to be, and another were where it 
really was, the two would seem to the naked eye to 
be one. But this was a distance entirely too great 
to be overlooked in astronomy. ♦ So it began to be 
suspected that there was still another planet outsidb 
of the orbit of Uranus, which by its attraction was 
causing this deviation. Two young mathematicians, 
Le Verrier, 1 of France, and Adams, 2 a student at 
Cambridge University, England, each without any 
knowledge of the other, attacked the problem. This 
was to determine whether this deviation of Uranus 
was caused by the attraction of an unknown planet, 
and, if so, where that planet must be. The problem 
was one of the greatest difficulty. Adams solved the 
problem first, and determined very nearly the true 
position of the unknown planet. But he failed to 
publish his results, and, although he sent them to the 
Astronomer Royal of England, they were not thought 
to be of sufficient importance to justify a search for 
the planet with a telescope. The next year, 1846, Le 
Verrier reached a result, which he published. This 
was found to agree closely with that of Adams, and 
search for the planet was at once begun at Cam- 
bridge at Adams's suggestion. But, while this search 
was going on, Le Verrier, who, like Adams, had no 
telescope at his command, wrote to Dr. Galle, 3 of Ber- 
lin, asking him to point his telescope to a certain spot 

1 See foot-note on page 65. 

2 John Couch Adams, born 1819. 

3 Dr. J. G. Galle (Gal'eh), 1812- , then assistant at the Berlin 
Observatory, now director of the Observatory at Breslau, Prussia. 



NEPTUNE. 185 



in the heavens, and to look for the new planet. Dr. 
Galle did so, and found the planet within less than one 
degree of the place designated. 

This most remarkable discovery in all the history of 
astronomy excited even greater enthusiasm than the 
discovery of Uranus. It made Le Verrier probably 
the foremost and most famous astronomer of the world, 
a place which he held for the rest of his life. In this 
wonderful discovery, Adams, prevented by no fault of 
his own from being the chief instrument in it, has re- 
ceived almost equal credit with Le Verrier; and their 
names are usually coupled together in the story of the 
discovery of the planet. 

180. Neptune's Satellite. — Only one satellite has been 
discovered revolving about Neptune. It is about as 
faint as the two smaller satellites of Uranus, and can 
be seen only in a large telescope. Its motion is still 
more retrograde than the motions of Uranus's satellites. 
It revolves about its planet from east to west. 

181. Observation of Neptune. — Neptune is never visible 
to the naked eye. It can be found only by having a 
telescope properly mounted, and pointing it to the 
place of the planet in the heavens as given in the 
Nautical Almanac. In an ordinary telescope it pos- 
sesses no interest, and very little in a large one. 

182. The View of the Heavens from Neptune. — From 
Neptune the sun's apparent diameter would be only 
about one-thirtieth of his apparent diameter to us, or 
about the same as that of Venus when she is nearest 
to us. Yet his light would be more than one hundred 
times as great as that of our full moon, insignificant as 
that would be compared with the light which we re- 
ceive from the sun (Art. 134). Owing to the smallness 

16* 



186 ASTRONOMY. 



and brilliancy of the sun, it is not likely that he would 
shew any disk at all to an observer on Neptune, but 
would be only an exceedingly brilliant star. Uranus 
and Saturn, and possibly Jupiter at times, are the only 
planets which could be seen by the naked eye from 
Neptune. All the planets within Jupiter's orbit would 
be too close to the sun to be seen. Although seemingly 
so far out towards the stars, they would not be appre- 
ciably brighter at Neptune than upon the earth. For, 
as we shall presently learn, the nearest star is so far 
away that the distance to Neptune, enormous as it is, 
becomes as nothing beside it. 

Are the Planets Inhabited? 

183. This is a question of great popular interest, and 
one that is often asked. But it is one to which astrono- 
mers pay very little attention, because of the impos- 
sibility of finding any satisfactory answer to it. The 
only heavenly body near enough to us to allow us to 
form an intelligent opinion about its being inhabited 
is the moon. And there the absence of air and water, 
and of any changes such as would be caused by our 
seasons, makes us certain that no life such as we know 
anything about exists. So far as we can tell, Mars 
most resembles the earth, and it has often been sup- 
posed that it may be inhabited. But upon Mars one 
would receive less than half as much heat from the 
sun as he gets upon the earth. This, unless modified 
by other circumstances, would reduce the temperature 
of Mars's surface far below zero, — a condition which 
of itself would, as it seems to us, make it impossible 
for life to exist there. As has been said, there is a 



ARE THE PLANETS INHABITED? 187 

strong probability that Jupiter and Saturn, and per- 
haps Uranus and Neptune, are still intensely hot, 
and, if so, incapable of sustaining life. If they have 
cold, solid crusts like that of the earth, the argument 
against the habitability of Mars would apply to them 
with increased force. But Prof. Tyndall 1 has pointed 
out that if the atmospheres of these distant planets 
were composed in part of certain vapors known to 
us, they would admit the sun's heat freely, but would 
prevent it from passing out again : just as window- 
glass allows the sun's heat to pass through it into a 
room, but allows very little of the heat of the room 
to pass out. Such an atmosphere would store up 
the sun's heat, and might make the distant planets 
inhabitable. 

Of the inferior planets, the one that seems to us most 
likely to be inhabited is Venus. As we never see 
Venus's surface through her dense atmosphere, we do 
not know how much resemblance she bears to the 
earth. But, as she receives twice as much heat from 
the sun as the earth gets, it seems to us scarcely pos- 
sible for life to exist there. Still, it may be that Venus 
has an atmosphere so dense as to protect her surface 
from the intense heat of the sun's rays. 

The question may be summed up by saying that if 
the earth as now constituted were suddenly put into 
the position of any one of the other planets, it seems 
certain that all life upon it would be speedily destroyed. 
As to whether varieties of life adapted to the different 
planets exist there, or whether these planets have con- 

1 John Tyndall, born 1820, Professor of Natural Philosophy in the 
Koyal Institution of Great Britain. He is one of the greatest of 
living scientists. 



188 ASTRONOMY. 



ditions and surroundings unknown to the earth which 
adapt them to such life as exists here, we do not know, 
and in all probability shall never find out. " Here we 
may give free rein to our imagination, with the moral 
certainty that science will supply nothing tending either 
to prove or to disprove any of its fancies." 



COMETS AND METEORS. 189 



CHAPTER VIU 



COMETS AND METEORS. 



We have so far considered, in the solar system, sun, 
planets, and moons. There yet remain to be men- 
tioned two other classes of bodies, comets and meteors. 
We will first treat of these separately, and then show 
an interesting connection between the two. 



Comets. 

184. General Appearance. — A comet is a body which, 
when visible to the naked eye, usually presents the 
appearance of a star with a tail extending out to one 





Fig. 43. — Telescopic Comet Fig. 44. — Telescopic Comet 

without a Nucleus. with a Nucleus. 

side. This tail nearly always points away from the sun. 
The smaller comets, such as can be seen only with a 
telescope, frequently have no tail at all, being simply 



190 ASTRONOMY. 



round masses of hazy light, either uniformly bright, or 
with a brighter spot near the centre. Figs. 43 and 44 
show two telescopic comets, one with and the other 
without a central condensation. 

185. Parts of Comets. — Comets are usually composed 
of three parts, the nucleus, the coma, and the tail. The 
nucleus is the star-like head in which a large portion 
of the light is concentrated. It is often bright enough 
to be seen in the daytime. In 1843 and 1858, in recent 
times, and in rnapy cases mentioned in history, comets 
have been distinctly noticed in the midst of the glare 
of sunlight. Though so bright, it is known that they 
have very little substance. While giving sometimes 
more light than the planet Jupiter, they are probably 
not one-millionth part as heavy. We know this from 
the slight effect they have on the planets when they 
approach them. A heavy body would attract a planet 
out of its path and change its orbit about the sun. 
Nothing of the kind has ever been noticed. It is sup- 
posed fhat on one occasion the earth passed through 
the tail of a comet. In 1770 a comet was discovered, 
which, in its approach to the sun, passed very close to 
Jupiter, and remained in its neighborhood for several 
months. But it did not seem to have any effect what- 
ever on Jupiter or his satellites, while the planet's at- 
traction changed the comet's orbit completely. 

The coma 1 is the envelope immediately surrounding 
the nucleus. It usually shades off from it, so that no 
distinct line of separation can be seen. Frequently it 
is composed of a series of circular bands of light, as 
in the drawing on the following page, made from a 



1 Latin for haw* 



COMETS AND METEORS. 191 

telescopic view, of CoggicCs comet. The nucleus and 
coma together make up the head. 

The tail stretches out from the coma, growing fainter 




Fig. 45.— Coggia's Comet, 1874. 



till entirely lost. It usually broadens as it recedes from 
the head. It also is extremely thin and rare. Even 
very faint stars can be seen through it. In the case of 



192 



ASTRONOMY. 




Fia. 46.— Donati's Comet, 1868. 



COMETS AND METEORS. 193 

the faint comets the tail is often not distinguishable at 
all, while in many of the brighter ones mentioned in 
history it stretched from the horizon to the zenith. 
Thus, we have an account of a comet in the year 134 
B.C. that " lasted seventy days; the heavens appeared 
all on fire ; the comet occupied a fourth part of the sky, 
and its brilliancy was superior to that of the sun; it took 
four hours to rise and four hours to set." Fig. 46 
shows the appearance of the tail of Donati's comet 
of 1858. What the faint comets lack in brilliancy 
and extent of tail they make up in number. Six or 
more telescopic comets are frequently discovered in 
one year, while but few great naked-eye comets are 
seen in a lifetime. Since the Christian era about five 
hundred comets have been recorded as visible to the 
unaided eye, while the number of telescopic comets 
seems to justify the saying of Kepler, that celestial 
space is as full of comets as the sea is of fish. 

186. Orbits of Comets. — Comets sometimes move in 
ellipses and sometimes in parabolas and hyperbolas. 1 
When the orbit is either of the latter curves, they 
approach the sun from outside space, we know not 
whence, swing around it, and go away never to return. 
When it is an ellipse, they move around the sun like 
the planets, returning again and again to the same 
point. The latter kind are called periodic comets, be- 
cause they have a regular period of revolution. Their 
reappearance can be expected, and the exact point in 
the heavens where they may be seen at a given time 
accurately calculated. The ellipses which the comets 

1 Ellipses and parabolas have been explained on page 33. A hy- 
perbola is a curve resembling in general appearance a parabola. It 
is not closed up like an ellipse. 

17 



194 



ASTRONOMY 



describe are, however, much flatter than those of the 
planets. Fig. 47 shows the orbit of Halley's 1 comet. 
Its point farthest from the sun is outside of Neptune's 
orbit, and its nearest 
about the distance of the 
earth. Such a comet, 
when far from the sun, 
will move very slowly, 
but as it approaches, its 
velocity, like that of a 
falling body, will in- 
crease. It will swing 
around the sun with im- 
mense rapidity, and fly 
off, moving continually 
more and more slowly. 

Many of the comets 
have doubtless been se- 
cured to the solar system 
as permanent members 
by the attraction of the 
planets. If a comet, 
coming in from outside 
space in a parabola, w^ere 
to pass in front of a planet, the planet's attraction 
might diminish its velocity, and change its orbit to an 
ellipse. The comet of 1770 thus came in from with- 
out, but Jupiter changed its orbit to a small ellipse, 
with a period of five and one-half yeare. It theiv per- 
formed two revolutions around the sun, when it again 

1 Halley, a friend of Newton, lived in England 1656 to 1742. 
Comets are sometimes named after their discoverers, and sometimes 
after the astronomers who calculate their orbits. 




Fig. 47.— Orbit of Halley's Comet. 



COMETS AND METEORS. 195 

approached the great planet, and had its orbit affected 
so that it has never since been seen. A number of 
comets revolve about the sun having their aphelia 1 at 
about Jupiter's distance, and it is supposed that these 
have all in past times been delayed by Jupiter, and so 
changed into permanent members of our system. 

187. Growth of Comets. — When a comet is first de- 
tected by a telescope as it approaches the sun, it usually 
appears as a round mass of hazy light of uniform 
brightness. Presently an increase of brightness shows 
itself in one point, which gradually grows into a 
nucleus. As it still draws nearer to the sun, an arch 
is seen partly to envelop the nucleus on the side 
next the sun; this growls longer and brighter, and 
finally the tail itself begins to grow away from the 
sun, reaching out farther and farther as the comet 
nears the sun, till at perihelion 2 it is longest and 
brightest. Sometimes a number of envelopes surround 
the nucleus, the whole presenting the appearance of 
a fountain shooting out towards the sun and then fall- 
ing away from it. This appearance is shown in the 
drawing of Coggia's comet, and also in those of the 
comet of 1861. The rapidity with which the tails of 
comets grow is wonderful; an increase of 35,000,000 
miles in a single day is recorded in one case. As the 
comet recedes from the sun it goes through its changes 
in reverse order, apparently drawing in its tail, and, 
finally, its envelopes, and returning to its condition of 
uninteresting uniformity. It has its brief day of light 
and activity, to be followed by a long night of dark- 
ness and rest. 

1 Points farthest from the sun. 2 Point nearest the sun. 



196 ASTRONOMY. 



188. What are Comets ? — This is a question to which 
it is difficult to give a complete answer. The spectro- 
scope seems to indicate that they are partly solid and 




Fig. 48.— Comet of 1861. 



partly gaseous. The nucleus is probably solid, and 
shines by reflecting the light from the sun. The coma 
and tail are gaseous, the former giving out light of its 



COMETS AND METEORS. 197 

own. This light is due largely to glowing carbon 
vapor, and resembles somewhat the flame of ordi- 
nary house-gas. It is a property of gases to expand 
indefinitely, unless held .in by a central attraction ; 
also it is known that in one case at least a comet van- 
ished from sight, and a shower of solid meteors took 
its place. We may, then, think of a comet as a solid 
nucleus or a collection of solid particles surrounded by 
a dense gaseous atmosphere in a state of great activity. 
This activity shows itself in the violent and rapid 
changes which are often noticed in the heads of comets. 
Sometimes a mass is thrown off from the nucleus and 
forms a separate body, which afterwards disappears. 
Sometimes there are jets seen extending from the nu- 
cleus towards the sun and on either side, w r hich usually 
curve backward into the tail. The sun seems to have 
a repulsive instead of an attractive force upon it, and 
the substance shot out into the tail represents so much 
waste matter to the comet. The tail is probably in the 
form of a hollow tube, as its edges often seem brighter 
than its centre. It is kept up by a constant flow of 
particles from the head. The enormous velocity of the 
head around the sun makes it improbable that the tail 
is rigidly attached to it, for if so it would be cast away 
by the great centrifugal force of the outer end. It is 
generally curved backward, thus showing that the par- 
ticles as they move out retain only the slower motion 
of the inner parts, and are therefore left behind. The 
bright comet of 1881 showed a great many of these 
changes and was carefully watched. 

189. Danger from Comets. — The ancients looked upon 
comets as the forerunners of war, pestilence, the death 
of kings, and all things evil. Their writings hence give 

17* 



198 ASTRONOMY. 



numerous and detailed descriptions of many of them, 
and astronomy is thereby the gainer. The real danger 
from a comet is that the large nucleus of one might 
strike the earth. This would produce such a blinding 




Fig. 49. — Comet of 1882, as seen on the morning of Sept. 30. A. Naked-eye view. B. 
Telescopic appearance of head. 

light and intense heat that all life on that side of the 
earth would be immediately destroyed. Most nuclei 
are, however, too small to produce such serious effects, 
and the chances of striking are so slight that we may 
as well dismiss all thought of its happening. 

190. Remarkable Comets. — Halley's comet was remark- 
able as being the first whose period of revolution was 
calculated or even suspected. After Newton had dis- 
covered the cause of the motion of the planets around 



COMETS AND METEORS. 199 

the sun, he predicted that comets would be found to 
obey the same laws, and hence that their regular re- 
turn might be expected. This set Halley to work to 
searching among the old records to find comets sepa- 
rated by uniform distances of time and whose orbits 
agreed. The comet of 1682 had just been an object of 
interest, and his search showed that in 1531 and 1607 
similar ones had appeared : he therefore felt justified in 
stating that they were successive returns of the same 
object, and that it would reappear about the early part 
of 1759. The event corresponded to the prediction. 
On the night of Christmas-day, 1758, it was first seen, 
and it reached its perihelion passage on March 12, 
1759. It returned again in 1835, and its next return 
will probably be in 1910. 

191. Encke's l comet has probably been studied more 
than any other. It was discovered at various times 
by different observers, among others by Caroline Her- 
schel, 2 though it was never suspected that it was the 
same object. Eucke, finding that its orbit was not a 
parabola, went into an elaborate investigation, and 
found that it revolved in an ellipse with the very short 
period of three and one-half years. Then, counting 
back, he found many records of its previous discov- 
ery. It has been watched several times since. 

This comet is of special interest to astronomers 
from the fact that its time of revolution is not uni- 
form. It arrives at its perihelion about two and one- 
half hours before the calculated time, so that its peri- 

^nk'eh, a German, 1791-1865. 

2 Caroline Herschel, the sister of Sir William Herschel, and his 
faithful assistant in much of his astronomical work. 



200 ASTRONOMY. 




odic time has diminished two days since it was first 
discovered. This indicates that it must be continually 
approaching the sun, as a small orbit and short periodic 
time always go together. The only plausible explana- 
tion of this fact is that the 
comet, being very light, is re- 
sisted in its motion by the 
ether which is supposed to 
fill all space. This resist- 
ance would tend to decrease 
the velocity and centrifugal 
force, and hence the comet 
would gradually fall towards 
the sun. It is a small tele- 

Fig. oO— Encke's Comet. 

scopio comet, usually, though 
not always, seen without a tail. Fig. 50 represents its 
appearance at one of its returns. 

192. Biela's l comet is remarkable for other reasons. 
Like Encke's, it was a small telescopic comet, and it had 
a period of about six and one-half years. Olbers 2 had 
called attention to the fact that in 1832 it would pass 
within 20,000 miles of the earth's orbit, though, as the 
earth would not reach the same point till a month later, 
no danger was apprehended by astronomers. Many 
other people, however, looked forward to the time with 
considerable anxiety. The comet came punctually as 
predicted, but no harm resulted therefrom. In 1846 
it again returned very close to the earth, and was care- 
fully studied. Much to the surprise of astronomers, 
while their telescopes were pointed at it, it began to 
divide into two comets, which gradually receded from 

1 Bie'la, a German, 1782-1856. 

2 Ol'bers, a German, 1758-1840. 



COMETS AND METEORS. 201 



each other as long as they were visible. When it came 
back in 1852 the division had increased, and measured 
one and one-quarter millions of miles. At the return 
of 1859 it would not be in a good position for observa- 
tion, but in 1866 it was expected with great interest, 
in order to notice what further changes might have 
taken place; but Biela's comet has never since been 
seen. No satisfactory explanation of its division and 




Fig. 51.— Biela's Comet, 1846. 

subsequent disappearance has ever been offered. Some 
other interesting facts in connection with this comet 
will be given in the section on meteors. The drawing 
represents its appearance just after the separation. 

193. Comet 6/ 1881. — This, the brightest comet since 
the spectroscope has been perfected, was observed more 
carefully than any other. The tail was nearly 40° long, 
and the head was as bright as, and larger than, a first- 

1 So many comets are now discovered that all of one year are named 
by the letters of the alphabet. This was the second one of 1881 



202 ASTRONOMY. 



magnitude star. Most of the phenomena described in 
paragraphs 187 and 188 were noticed in this comet. 
At one time a separation in the nucleus led astronomers 
to think that it would follow the example of Biela's 
comet (see Fig. 51); but by the following night the 
smaller part had disappeared. Very frequent changes 
were observed by the telescope, in the shape of the jets 
and envelopes around the nucleus. Many drawings 
were made of it, it was carefully studied by means of 
the spectroscope, and (for the first time with comets) 
a photograph was taken of it. 

Comet b, 1882. — This comet came very unexpect- 
edly, and was seen about September 17 near the sun 
in broad daylight. It then passed to the east of the 
sun, and appeared as a brilliant object to the naked 
eye for two months. In the telescope its nucleus was 
observed to undergo rapid changes, dividing into two 
or more parts, which afterwards seemed to come to- 
gether again. An extension of the tail several degrees 
from the nucleus towards the sun was visible at one 
time. The tail as seen by the eye was about 15° long 
and 2° broad. 

When its orbit was calculated it was found to agree 
quite closely with that of comets which had appeared 
in 1843 and 1880, but a further computation showed 
that it moved in a vast ellipse, taking (according to 
one computer) about 800 years to go around. This 
would carry it to a distance from the sun about five 
times as far as is Neptune. At its perihelion it passed 
within half a million miles from the solar surface, while 
at aphelion it will be something like 16,000 times as 
far away. The breadth of the ellipse would be about 
one-sixtieth its length. 



COMETS AND METEORS. 203 

While it is not the same comet as those of 1843 and 
1880, yet it is hardly likely that the agreement of their 
orbits is accidental. Some unknown connection prob- 
ably exists among them. 

The Lexell-Brooks Comet — A recent comet with an 
interesting history is this one. It first appeared in 
1770, and an orbit of five and a half years was com- 
puted for it by Lexell. It was never seen again till 
1889. In the mean time it seems probable it had had 
a devious course. Jupiter, which had first brought it 
into the solar system by changing its orbit from a par- 
abola to a small ellipse, changed it again to a large 
ellipse, and now, after more than a century, has brought 
it again into visibility. It is not unlikely that it will 
again approach Jupiter, w r hen some other change, as 
yet unknown, may be expected. 

194. How to Look for Comets. — From what has been 
said we may infer that new and interesting comets may 
be expected at any time and in any part of the heavens. 
The number of those whose return can be predicted is 
very small as compared with the number of new ones 
which may be discovered. Prizes of medals or sums 
of money have been offered at various times for the 
discovery of comets, and this and the honor connected 
with it have led many to search for them with great 
perseverance. The work does not need large telescopes, 
nor does it require any skill which an ordinary person 
with a good eye cannot acquire. The instrument used 
should have a low power, such a one as would magnify 
about twenty-five times, and it should have a large 
field of view. The comet is detected partly by its ap- 
pearance and partly by its change of place among the 
stars. Judging by appearance alone, a nebula or cluster 



204 ASTRONOMY. 



might be mistaken for a comet. But a little watch will 
show whether there is any change of position with refer- 
ence to the neighboring stars. If a discovery is made, 
it should be immediately telegraphed to some observa- 
tory. American observers have divided the sky into 
zones, for systematic comet-seeking. They are sup- 
plied with catalogues of clusters and nebulae, and, be- 
sides their announcements of discoveries, report their 
work monthly. 

Meteors. 

195. General Remarks. — Every one is familiar with 
" shooting-stars." This name describes their general 
appearance, but in reality they are very different from 
stars. The stars are all at an immense distance from 
us, while meteors are in our atmosphere. Stars are 
very large bodies, while meteors are quite small. No 
instance of visible motion in a star has ever been 
noticed, nor is it likely ever to be. When we use 
the name " shooting-star," we must remember the great 
difference which exists between them and the real 
stars. 

Upon almost any clear, moonless night we can see 
meteors. It has been calculated that the average 
number visible is about five an hour. Sometimes it 
vastly exceeds this. As many as 30,000 an hour is 
given as the number seen on several occasions. When 
a watch for them is carefully maintained, it is noticed 
that on certain nights of the year they are especially 
numerous. About the 10th of August and the 14th 
of November a person cannot fail to notice a large 
number during any clear and moonless night. 

196. What are Meteors ? — Meteors are small, cold. 



COMETS AND METEORS. 205 

solid bodies, sometimes probably merely misty clouds of 
light matter, which are revolving about the sun entirely 
independently of the earth. Sometimes, however, as 
the earth is moving on its orbit with tremendous ve- 
locity, it approaches some of these little bodies. The 
earth's atmosphere probably extends a hundred or more 
miles in all directions from its surface, and the meteors 
strike it with great energy. Now, we know that heat 
is produced when a nail is struck with a hammer; just 
so when a meteor, probably with a great velocity of its 
own, comes in contact with the atmosphere moving to 
meet it at a rate of 66,000 miles per hour, it is made 
so hot that it burns. It keeps on moving until entirely 
consumed, and thus we see the blazing streak across 
the sky. The train which is sometimes seen to follow 
it and to float away like a light cloud is the red-hot 
ashes, which gradually and imperceptibly fall to the 
earth. Sometimes the concussion is so violent as to 
break the meteor into fragments, and we hear a loud 
report and see the flying masses in their separate tracks. 
If the body be very large and bright, and seem to pass 
a long way through the atmosphere, and to approach 
very near the earth, it is called a, fire-ball; and if still 
larger, so as not to be consumed by the time it reaches 
the earth, it is usually called an aerolite} Many aero- 
lites have been seen to fall which have afterwards been 
picked up and examined. They show the indications 
of intense heat in their glazed surfaces, which have evi- 
dently been molten. This has been done in their pas- 
sage through the atmosphere. But, besides this, when 
they are carefully examined with a microscope their 



1 A'erolite, a stone from the air. 
18 



206 ASTRO XO MY. 



whole interior structure is found to be crystalline : this 
proves that at some time they must have been either 
wholly molten or gaseous. They are usually largely 
composed of iron, but often have a number of other 
substances in connection with it, though they contain 
no elements which are not found on the earth. It is 
interesting to know that the matter that comes in to us 
from outside space contains the same substances which 
exist here. We have seen that the sun is made up 
largely of terrestrial elements, and we shall find the 
same to be true of the stars. It is probable that on 
the earth we have specimens of nearly every element 
that exists in the universe. 

It is calculated that meteors begin to burn at the 
height of about seventy miles from the earth, and the 
smaller and more inflammable are usually consumed 
after a course of about forty miles through the air. 
They are mostly small, usually about a grain in weight, 
though many are larger. 

197. But how are these little masses arranged in 
space? It is known that they are not uniformly dis- 
tributed, but that they are collected into rings about 
the sun. These rings are not circular, but are elliptic, 
like the orbit of a comet, having the sun in a focus. 
There are many millions of meteors in every ring, and 
they follow one another in this ring round and round 
the sun. They are in reality very minute planets which 
have been crowded together and are performing their 
revolution around the sun in company, every one, no 
doubt, much influenced by the attractions of the others. 
The meteors are not always arranged in the rings 
uniformly, but are collected in groups, with compara- 
tively barren spaces between them. 



COMETS AND METEORS. 207 

198. November and August Meteors. — While the me- 
teors themselves revolve about the sun, the ring, con- 
sidered as a whole, retains the same position all the 
time. Hence if the earth passes through the ring in 
one of its revolutions, it will pass through it on each 
succeeding revolution at the same time of year. Every 
year, accordingly, when the earth reaches the point of 
its orbit which intersects the ring, unusual displays 
may be looked for. If the meteors were distributed 
uniformly along the ring, the same number would be 
seen every year; but if there were one main bunch 
in the ring and the remainder of the meteors were 
more thinly scattered along, the display would be most 
striking when the earth encountered this main body. 
Since the meteors have a regular time of revolution 
about the sun, we might expect these striking dis- 
plays to recur at regular intervals, when the mam 
bunch comes around. 

Such is the case. Every year on the 14th of No- 
vember, and on several nights on both sides of this 
date, unusual showers of meteors are observed ; while 
at intervals of about thirty-three years the display is 
remarkably brilliant. One of these occurred on No- 
vember 12, 1799, when Humboldt 1 saw it in South 
America, and described it as follows : " Thousands of 
bodies and falling stars succeeded each other during 
four hours. From the beginning of the phenomenon 
there was not a space in the firmament equal in extent 
to three diameters of the moon which was not filled 
every instant with falling stars." On November 13, 
1833, the most brilliant display of meteors on record 

* Hum'bolt. A great German scientist, 1769 to 1859. 



208 ASTRONOMY. 



occurred. It was visible all over North America. The 
whole heavens seemed on fire, and the greatest con- 
sternation prevailed among ignorant people. Again 
on November 14, 1866, the shower returned, this time 
visible in England. It was observed with care, and 
about 8000 were counted in one night at the Green- 
wich Observatory. As the meteor-bunch occupied 
some time m passing the point of contact with the 
orbit of the earth, the display appeared for several 
years succeeding this. 

The explanation of these regularly returning showers 
has already been indicated. Every year, on the 14th 
of November, the earth in the course of its journey 
around the sun passes through this meteoric ring, and 
we have the yearly display. But the meteors which 
constitute the ring are not uniformly distributed along 
its course. There is one main collection of them 
and probably several smaller ones. Moreover, these 
meteors are themselves revolving about the sun, com- 
pleting a revolution in thirty-three and one-fourth 
years. Hence at intervals of this time the earth en- 
counters this main swarm and ploughs a path through 
it. The bodies are attracted to the earth, which is also 
advancing to meet them, and the collision and friction 
with the atmosphere give us the splendid displays of 
shooting-stars. Fig. 52 shows how the earth's orbit 
comes in contact with the November meteor rings. 
When this point is reached by the earth, we have the 
yearly display. If a bunch of meteors happens to be 
passing at that time, the display is magnified. 

Meteors are also abundant on and about the 10th 
of August. Unlike the November meteors, these are 
about equally conspicuous every year. They are prob- 



COMETS AND METEORS. 209 

ably distributed along their ring uniformly, so that 
their period of revolution has not been certainly de- 
termined. 



Fig. 52.— Orbits of August and November Meteors. (From Schellen's Spectrum 

Analysis.) 

199. Radiant Point — When the paths which a show- 
er of meteors describes in the heavens are marked on 

18* 



210 ASTRONOMY. 



a celestial map, it is found that if produced back- 
ward they would all intersect nearly in one point. This 
is shown in Fig. 53. They all seem to radiate from this 
portion of the heavens, and hence the name radiant 
point is given to it. The radiant point for these meteors 
is in the constellation Orion. They are therefore called 
Orionids. The radiant point of the August meteors is 
in Perseus, and they are hence called Perseids. The 
November meteors are Leonids. It must not be inferred 
that the meteors actually move in divergent lines. 
Their paths are really parallel; but just as railroad- 
tracks seem to approach each other as they recede from 
an observer, so these parallel lines appear to radiate 
from a common point. Just in the radiant point the 
meteors are seen without any motion, because they are 
directly approaching us. In the case of the great No- 
vember showers, it is stated that in Leo the sky seemed 
phosphorescent from the large number of meteors 
shooting directly towards the observer. 

200. Sow to Watch for Meteors. — Observations on 
meteors are especially adapted to young astronomers, 
as they require no expensive implements and no skill 
which cannot be easily acquired by a patient watcher. 
It is important that the position of the radiant point be 
accurately determined, as this distinguishes the meteors 
of one group from those of another, and is also neces- 
sary to calculate their orbit. The following is the 
method to be employed. Procure a reliable map of the 
heavens, 1 and, spreading over it a sheet of thin, partly 

1 A planisphere set to the middle of the watch is convenient fot 
ordinary work. 



COMETS AND METEORS. 211 

transparent paper, mark on it a, number of the prin- 
cipal stars, being especially careful to place those near 
the radiant point with great accuracy. Mark also the 
points of the compass around the horizon. Now be- 
come very familiar with the map thus formed, so that 
it will be easy to find any part of the heavens and any 
star quickly. Whenever a meteor is noticed, note with 
great accuracy its path in the sky, and transfer it to the 
map, indicating the length of the track and the direc- 
* tion of the motion as in Fig. 53. Notice also the 
brightness of the meteor as compared with Jupiter, 
a first-magnitude star, etc., and write the same by the 
side of the mark. Eecord also the exact time as 
nearly as may be, the color of the meteor, whether it 
left a streak behind it or not, whether its course was 
slow or rapid, and any other interesting facts connected 
with it. Look out especially for meteors with short 
tracks near the radiant point, and record them with 
especial care; and if a meteor blazes out without any 
track at all, its position should be exactly found. When 
the watch is over, trace back to their intersection the 
various paths belonging to a common system, and de- 
termine the right ascension 1 and declination 1 of this 
point. Preserve the map for future reference. Should 
there be a great shower, it is well for one person to 
count the number seen in each five minutes of the 
watch, while another records the principal ones. The 
following table shows the times in the year when the 
earth passes through some of the principal meteor 
rings, and the radiant point for each : 

1 See page 42. 



212 



ASTRONOMY. 



Date. 


Radiant Point. 


Name. 


R. A. 


Dec. 


January 2-3. 
April 19-20. 
July 27-31. 
August 9-11. 
October 18-20. 
November 12-14. 
November 27. 
December 9-12. 


232° 

272° 

337° 

44° 

89° 

149° 

25° 

105° 


+49° 
+35° 
— 6° 
+56° 
+15° 
-^23° 
+43° 
+31° 


Quadrantids. 

Lyrids. 

Aquarids. 

Perseids. 

Orionids. 

Leonids. 

Andromedes or Bielas. 

Geminids. 



Instead of a map a slated globe may be used. This 
is more reliable than a map, but more expensive. It 
may be prepared by marking on it with white paint the 
circles of right ascension and declination, and the posi- 
tions of the principal stars down to the fourth magni- 
tude. The meteor tracks may be marked on this with 
a soft slate-pencil or sharp chalk and afterwards trans- 
ferred to a piece of paper. In all cases it is well to 
rule a table something like the following, and record 
all prominent meteors : 



Time. 


Beginning 
of Track. 


End of 
Track. 


Bright- 
ness. 


Rate of 

Motion. 


Color. 


Remarks. 




R. A. 


Dec. 


R. A. 


Dec. 


November 14, 
1881, 4.20 a m. 


153° 


+37° 


228° 


-f68£° 


1st mag. 
star. 


Rapid. 


Yellow. 


Leonid; not 

very well 

observed ; 

streak. 



Fig. 53 shows a map of Orionids made at Haverford 
College Observatory on the morning of October 19, 
1881, and is the result of an hour's watch. The neigh- 
boring hours were quite as fruitful of meteors as this, 
but the addition of any more would only confuse. The 



COMETS AND METEORS. 



213 



Sukh ft, LfO 




Fig. 53.— Orionids of October 19, 1881. 



general divergence of the tracks from a point marked 
by a small circle is quite noticeable. Some of them do 
not seem to radiate strictly from it, but this is due 
probably to errors of recording, a large portion of 
which are unavoidable. Some of them belong to other 
showers than the Orionids, and their presence with the 
others is only accidental. All are given to show the 
appearance of such a map at the close of a watch. 

201. Zodiacal Light — This name is given to a faint 
light which may be seen after sunset on clear evenings 
of the winter and spring. It is triangular in appear- 



214 ASTRONOMY. 



ance, its base being on the western horizon, and its 
greatest length extending back along the path of the 
sun; it may, on favorable evenings, be observed to 
extend nearly to the meridian, where it fades away so 
that no distinct outline may be noticed. Some people 
with very good eyes claim to be able to see it all the 
way to the eastern horizon. It really exists in the 
summer and autumn as well, but in our latitude the 
ecliptic 1 lies so near to the horizon at these times that 
the light cannot be easily distinguished. It is then, 
however, visible in the morning just before sunrise on 
the other side of the sun. It is not certainly known 
what it is, but the most probable theory is that it is 
composed of an immense number of meteoroids, re- 
flecting the sunlight, and which are so small that their 
united lustre is barely distinguishable. They surround 
the sun on all sides, revolving about him like little 
planets, and frequently fall to his surface, thus assisting 
in keeping up his heat and light, as explained on page 
62. The light should be looked for from a half-hour 
to an hour before sunrise or after sunset. 

Relation between Comets and Meteors. 

202. Prof. Newton, of Yale College, and Prof. Adams, 
of England, entered into an elaborate investigation to 
find the orbit of the November meteors. They based 
their work on the old records of former displays and 
the observations of the shower of 1866. The result of 
their labors was to lay down very accurately the size 
and position of the orbit. When this w r as done it was 

1 gee page 39. 



COMETS AND METEORS. 215 

found that it agreed almost exactly with that of a small 
comet commonly known as Comet L, 1866, or Temper s 
comet, which also had a period of about thirty-three 
and one-fourth years, and which returned to perihelion 
in the early part of the same year in which the brilliant 
meteoric display occurred. It was thus found that the 
main group of November meteors followed around in 
the orbit of the comet ; that the earth met the comet 
about ten months before the meteoric swarm ; and that 
the comet led the way round and round the sun, with 
the swarm immediately and continually following it. 
Fig. 55 shows the identity of the two orbits. 

But this is not an isolated instance. It was soon 
found that the August meteors and Comet III., 1862, 
had identical orbits, and, later, a similar relation was 
found to exist between the Lyrids of April 20 and 
Comet I., 1861. Another interesting case is that of 
Biela's comet. We have said (page 201) that in 1866 
it was expected, but that it never appeared. The next 
return would have been in 1872. On the night of 
November 27 the earth and the comet, it was calcu- 
lated, would be at the same point at nearly the same 
time. But there came, instead of the comet, a shower 
of meteors. They rained down on England at the rate 
of over ten thousand an hour ; they brightened up the 
earth and sky, and many an observer recorded the fact 
that they all radiated from the same point in the con- 
stellation Andromeda, and that point was just where 
the comet was expected to come from. The comet had 
gone no one knows where, but a swarm of meteors had 
assumed its place. Every year, on the 27th of No- 
vember, the shower may be seen, and its brightness 



216 ASTRONOMY. 



increases as the time for the regular return of Biela's 
comet approaches. 

These coincidences cannot be attributed to chance. 
There must be some connection between comets and 
meteors, but what it is astronomers have not certainly 
determined. We should keep on observing facts, trust- 
ing that soon the mystery will be thoroughly cleared 
up. It is probable that the solid portion of the comet 
has been broken up by some internal convulsion, and 
that the meteors are the fragments. 



PART II. 

THE SIDEREAL UNIVERSE. 



CHAPTER I. 



THE CONSTELLATIONS. 



203. Introductory. — So far we have kept within the 
limits of the solar system. We have been struck with 
the immense intervals of space which separate its dif- 
ferent members. The distance to the moon is greater 
than anything we can conceive of, yet it is but a trifle 
when compared with the distance to the sun ; but even 
the earth's orbit seems small when we think of the enor- 
mous length of the path which Neptune passes over in 
each revolution about the sun. We now are about to 
consider bodies whose distance is so great that the 
huge orbit of Neptune seems but a point in comparison. 
When we gain some familiarity with them, the solar 
system, great as it is, will seem to us like a little com- 
pany of orbs, near at home, clustered together in infi- 
nite space ; a very insignificant portion of the whole 
universe. These distant bodies are the stars and the 
nebulce. Taken as a whole, they constitute the sidereal 
system. This system embraces all the heavenly bodies. 
The solar system is a part of it, to us a very conspicu- 

19 217 



218 ASTRONOMY. 



ous part, but, compared with the whole, quite diminu- 
tive. We must not include under the name stars the 
various members of our solar system. 

Though Jupiter and Venus and the other planets 
resemble stars to the eye, they really differ widely. 
The planets revolve around the sun like the earth ; the 
stars do not. The planets are comparatively close to 
us ; the nearest star is more than seven thousand times 
as far from the sun as is Neptune. The planets shine 
by the light which they reflect to us from the sun ; the 
stars give out their own light. 

204. What are Stars ? — The stars are suns. They give 
out light and heat like the sun. As soon as we come 
to treat of the distance to the stars we shall see that it 
would be impossible to consider that they receive the 
light which they send us from the sun as the planets 
do. They must be hot and glowing bodies themselves, 
some of them as large and as bright as the sun, and 
some of them probably much larger and brighter. If 
we were to look at the sun from their distance, it would 
appear to be a little point of light as they do. The sun 
is a star. We must consider space to be occupied with 
a countless number of suns scattered very thinly through 
it; and, though we have never seen any worlds sur- 
rounding any sun but our own, it is very probable that 
they exist, and that each star is the centre of a system 
to some extent resembling the solar system. There is 
another proof of the fact that the stars are like the sun. 
The spectroscope tells the same story of both : they 
both consist of a glowing mass, the light from which 
shines through a gaseous, less bright atmosphere, and 
the materials of which this atmosphere consist are 
largely the same in all. 



THE CONSTELLATIONS. 219 

The ancients had in general very incorrect ideas as 
to what the stars were. They were variously supposed 
to be studs nailed to the celestial sphere; fires which 
were nourished by the igneous matter which streamed 
out from the centre of the earth; luminous stones 
whirled up from the earth ; breathing-holes in the uni- 
verse. Pythagoras had a more exalted idea of them, con- 
sidering them to be worlds having land, water, and air. 

205. Constellations. — In very remote antiquity the 
heavens were divided up into groups of stars which 
were called constellations. Names were given to some 
of these, probably on account of their fancied resem- 
blance to certain animals and other objects, though it 
is difficult to see any such resemblance now. The 
seven stars commonly called the Dipper are a part of 
the constellation "Great Bear" yet the arrangement of 
the stars in this constellation does not seem to suggest 
anything of the kind. Other names, as Hercules, were 
probably given for the sake of honoring their deities 
or great men. Astronomers have found it convenient 
to retain these names to mark certain portions of the 
heavens. Thus, Leo does not now mean a great lion 
or anything resembling one, but a definite section of 
the celestial sphere containing certain stars. 

206. Names of the Stars. — The heavens being thus 
divided up into small areas, with a name for each, i1 
remained to adopt some plan for distinguishing one 
star from others in the same constellation. The method 
now in use was suggested by Bayer 1 in 1603. The 
brightest star of the constellation is named by prefixing 



1 Bayer, a German astronomer and Protestant preacher, 1572 to 
1660. 



220 ASTRONOMY. 



the first letter of the Greek alphabet to the genitive case 
of the name of the constellation. Thus, the brightest 
star in the Great Bear is a Ursce Majoris. The brightest 
in Leo is a Leonis. The other Greek letters and the Ro- 
man letters then follow somewhat indiscriminately; and 
if this does not exhaust the stars of any constellation, the 
remaining ones are numbered. This order is not strictly 
correct; the namers sometimes misjudged thebrightness 
of the stars, and it is probable that the splendor of some 
of them has changed since the names were given. Thus, 
/? Orionis is in general brighter than « Orionis, but 
sometimes the latter greatly exceeds it in light. Be- 
sides this, the stars in the different constellations have 
been independently numbered, so that a single star is 
sometimes designated by its letter as well as by its num- 
ber. This double system often causes confusion. Many 
hundreds of the stars had received special names from 
the ancients, particularly from the Arabians ; but the 
inconvenience of remembering these has kept them 
from being extensively used, except in the case of a 
few of the most conspicuous of them. Thus, a Bootis 
is Arcturus, a Lyrse is Vega, etc. 

207. Magnitudes. — For convenience of classification 
the stars have been divided according to their bright- 
ness into magnitudes, the first-magnitude stars being 
the brightest. Stars of the first five magnitudes can be 
seen with the naked eye, and on favorable nights many 
stars of the sixth magnitude may also be seen. The 
number in each of the first six magnitudes is approxi- 
mately given in the following table : 



1st . 


. 20 


4th. 


. 450 


2d . 


. 65 


5th. 


. 1100 


3d . 


. 200 


6th. 


. 4000 



THE CONSTELLATIONS. 221 

It thus appears that there are about 6000 stars which 
under the most favorable conditions can be seen by the 
naked eye. Some of these are never above the horizon 
in middle latitudes, and only a part of the remainder 
are above at any given time. Many eyes cannot see 
the fainter ones of the sixth magnitude, so that 2000 
will cover all commonly seen, by most people, at any 
one time. Telescopes reveal them by the millions. Fig. 
54 shows the appearance of a portion of the heavens as 
seen by a telescope. There are not more than two 
stars here visible to the naked eye. 

Sir "William Herschel gives the following table as 
representing the light given out by a star of the differ- 
ent magnitudes, an average sixth-magnitude star being 
taken as unity : 



6th 


. 1 


3d . 


. 12 


5th 


. 2 


2d . 


. 25 


4th . 


. 6 


1st . 


. 100 



As a general rule, an average star of any magnitude 
is about two and one-half times as bright as an average 
star of the magnitude next fainter. 

It must not be supposed that all stars of the same 
magnitude are equally brilliant. There are all grades 
of brightness, from Sirius, the light from which is esti- 
mated to be 234 times as great as that from a sixth- 
magnitude star, to those so faint as scarcely to be vis- 
ible. There is no distinct line to be drawn between 
the different magnitudes. A star lying between the 
fourth and fifth magnitudes, for example, might be 
considered to be a faint star of the fourth by some 
astronomers and a bright star of the fifth by others. 
These intermediate stars are often designated by deci- 

19* 



222 



ASTRONOMY. 



mals. Thus, magnitude 4.8 would mean nearer the 
fifth than the fourth. 

208. The following list contains the names of twenty 
of the most brilliant stars of the heavens arranged 
nearly in the order of brightness ; those in italics are 
never seen in the latitude of New York : 



a Canis Majoris, or Sir'ius. 

a Argus , or Cano'pus. 

a Gentauri. 

a Bootis, or Arctu'rus. 

(3 Orionis, or Ri'gel. 

a Aurigse, or Capel'la. 

a Lyrae, or Ve'ga. 

a Canis Minoris, or Pro'cyon. 

a Orionis, or Betel/geuse. 

a Eridani, or Acher'nar. 



a Tauri, or Aldeb'aran. 

J3 Centaur l, 

a Cruris. 

a Scorpii, or Antar'es. 

a Aquilae, or Altair 7 . 

a Virginis, or Spi^a. 

a Piscis Australis, or Fo^ialhaut. 

Cruris. 

(3 Geminorum, or PoFlux. 

a Leonis, or Reg / ulus. 



These should all be found on a celestial globe or 
map, and then in the heavens. They will serve as val- 
uable starting-points from which to locate the fainter 
stars. To give examples of stars of the lower magni- 
tudes, we will go over the stars of the Dipper : 

The brightest of the pointers is of the second mag- 
nitude ; the other pointer is of the third ; the star next 
to this is also of the third; the star which joins the 
handle of the dipper to the bowl is of the fourth ; the 
next star in the handle is of the third ; the next, of the 
second; and very close to it is one of the fifth; the 
last one in the handle is of the third. On a moonless 
night, with a clear atmosphere, there are three little 
stars of the sixth magnitude which may be seen near 
the star at the end of the handle of the dipper, and in 
the direction of the pole-star. 

Star Maps and Stellar Photography. — It is necessary 



THE CONSTELLATIONS. 223 



for any one who has work to do with the stars to have 
accurate maps of the heavens. Several of these, em- 
bracing all visible stars and some others, have been 
made, and are very good. The best way to make these 




Fig. 54.— Part of the Constellation Cygnus, as seen with a Telescope. 

maps is by photography. The telescope is pointed to 
a portion of the heavens, and the image is allowed to 
fall on a photographic plate instead of the eye. Every 
star leaves its impression in just the right relative po- 



224 ASTRONOMY. 



sition ; and it is a curious fact that stars too faint to be 
seen by the eye will impress their images on the plate 
if the exposure is continued long enough. 

A company of astronomers, scattered over the world 
at eighteen different observatories, is now at work to 
secure maps of all parts of the heavens. Several years 
will be required to perform the task. When com- 
pleted the most accurate and exhaustive star-charts 
ever made will be at the service of astronomers. About 
twenty thousand plates will be made of the heavens. 
Another such series, taken a few years later, will show 
very satisfactorily any changes in the relative position 
of the stars. 

209. The Milky Way, or Galaxy. — This is a ring of 
hazy light, which seems to encircle the earth, and is 
visible on moonless nights. By the telescope it is 
shown to consist of millions of stars clustered together. 
They are either so small or so distant that we cannot 
see them separately by the unassisted eye. The Milky 
Way partakes of the diurnal motion of the heavens, and 
is therefore seen in varying positions at different times. 
But it never changes its place among the stars. The 
telescope shows that nearly all the faint stars are ^clus- 
tered in or around it. We see but few near its poles, 1 
but as w r e approach they increase in number very rap- 
idly. A careful observer can see this — less conspicu- 
ously, though — with regard to the brighter stars. A 
large majority of all the stars are clustered in or near 
the plane of the Milky Way. 

In the Galaxy itself the stars are not distributed uni- 
formly, but grouped in large or small clusters, with 



1 Points 90° from it. 



THE CONSTELLATIONS. 225 

blank spaces between them. The edge is jagged and 
irregular, and wherever there is an outlying streamer 
there the lucid stars seem also to be conoreerated, so as 
to suggest some connection. At one point it divides 
into two parts, which afterwards join each other. 

210. Distances of the Stars. — If a person were walking 
past a grove of trees, they would seem to him to be con- 
tinually changing their places among one another; the 
trees nearest him would appear to move backward, with 
respect to those beyond ; two trees which were in a line 
with him at one instant w T ould seem to separate as he 
advanced ; others would appear to approach one another. 
Now, the earth is sweeping through space around the 
sun, passing every six months from one side of its orbit 
to the opposite. These points are separated from each 
other about 186,000,000 miles. It would seem, then, 
that the stars ought to change their apparent places 
among one another just as the trees do ; in other words, 
that the stars ought to show some parallax. Evidences 
of this were sought with great care for a long time 
without any success. The stars were so distant that 
any change of position was inappreciable. With in- 
struments capable of greater precision a very small 
parallax has in modern times been detected for certain 
stars, though in no case has it been found to equal one 
second of arc. Let us consider the meaning of this. 
Suppose that an observer were on the nearest star, with 
a telescope steadily pointing to the earth as it moved 
from one side to the other of its immense orbit. The 
direction of the telescope at the beginning of the watch 
would deviate from that at the end by an angle of one 
and a half seconds. The radius of the earth's orbit 
would subtend an angle of only three-quarters of one 



226 ASTRONOMY. 



second; if it were a luminous rod, it would appear no 
longer than a foot-rule would at the distance of about 
fifty miles. A calculation shows that the distance to 
this star is about 25,000,000,000,000 miles ; but there 
is no advantage in expressing a distance as great as 
that to the stars in miles. The number is utterly in- 
conceivable. If we take the distance to the sun as our 
unit, or measuring-rod, there would be 264,000 such 
units in the line joining us to the nearest star. Such 
distances are usually measured by the time it takes 
light to pass over them. Light moves with a velocity 
of about 186,000 miles per second. It would take it 
about one and a third seconds to pass between the earth 
and the moon ; in a little over eight minutes it comes to 
us from the sun ; but to reach us from the nearest fixed 
star over four years are required, while its time in com- 
ing from many of the stars is measured by centuries. 

The nearest known star, a Centaury in the Southern 
hemisphere, is a very bright one. The next one, 61 
Ci/gni, is only of the sixth magnitude. It is not found, 
as a rule, that the brightest stars are the nearest. 
Sirius, the most brilliant star of the whole heavens, is 
three times as far away as 61 Cygni. This requires us 
to suppose that the stars are of different sizes. Sirius 
must be very much larger than 61 Cygni to give out so 
much more light at a greater distance. Knowing the 
relative distances from us of Sirius and the sun, and 
the light given by each, we can calculate that, sup- 
posing their surfaces to be equally bright, Sirius must 
have about fifty-six times as much surface as the sun, 
and hence over seven times as long a diameter. 

Though the stars are such large bodies, they show no 
diameters in the most powerful telescopes. They are 



THE CONSTELLATIONS. 227 



merely points of light, brighter, but no larger, than 
when seen by the eye. This is shown by an occulta- 
tion of a star by the moon. (Page 139.) 

211. Motions of the Stars. — The ancients called the 
stars fixed, because they did not seem to change their 
places among one another as the planets did. By com- 
paring their relative positions now T with what they were 
when the first star catalogues were made, it is found that 
many of them have very considerable motion. This 
motion of the stars among one another is called proper 
motion. In the course of an immensely long period the 
constellations will be distorted. As an illustration of 
how r this is possible, we may take the case of the Great 
Bear. Of the seven principal stars forming the Dipper, 
it has been found that five are moving in parallel lines 
and with equal velocities. Their relative positions 
will therefore be preserved ; but the two pointers are 
moving in opposite directions. After centuries have 
elapsed they will cease to point to the pole-star, and 
the Dipper will change its shape. 

This motion can be detected only by very delicate 
measurements. It is probably extremely rapid, but 
the great distance makes it appear to be very minute. 
We must also remember that the stars are not bodies 
at the same distance from us, set in the surface of the 
same sphere, and moving in that surface, but are at 
varying distances, and moving in all directions, towards 
us and away from us, as well as siclewise. The latter we 
might hope to detect, but, until recently, motion to us 
or away from us appeared hopelessly un discoverable. 
But the spectroscope has enabled us to solve the diffi- 
culty, and we can now tell with reasonable accuracy 
which way nearly all the bright stars are moving. 



228 ASTRONOMY. 



Like the other stars, the sun has a proper motion, 
carrying with it the whole solar system. This has been 
proved by the apparent motion of the stars. Let us 
recur to our illustration of the grove. Suppose the 
observer to be directly approaching it : the trees would 
then appear to be separating from one another. If he 
were moving away from it, the trees would seem to 
close up. Now, if in any part of the heavens the stars 
are apparently opening out, and in the opposite portion 
closing up, it is evidence that we, in common with the 
rest of the solar system and the sun, are approaching 
the centre of the former part. This point has been 
calculated by various astronomers, and they agree in 
placing it in the constellation Hercules. 

212. Colors of the Stars. — It is easily observed that 
the stars vary in color. Any one carefully noticing 
the colors of Sirius and Betelgeuse, for example, would 
not fail to see a striking difference. They are usually 
either blue, white, yellow, or red. Of the stars which 
have a blue or green tinge, Sirius, Vega, and Rigel 
may be mentioned; of the white stars, Regulus and 
Polaris; of the yellow, Capella and the sun; of the 
red, An tares and Betelgeuse. 1 There are also a num- 
ber of faint telescopic stars which are of a deep blood- 
red color ; these are usually remarkable for changes in 
their brightness. The student should carefully exam- 

1 J. Norman Lockyer, a noted living English astronomer, has pro- 
pounded the theory that the colors of the stars indicate the intensity 
of their heat. Just as a piece of metal goes through its changes from 
red-hot to white-hot as the temperature is raised, so the red stars are 
the least heated of all visible stars. Then follow in order the yellow, 
the white, and the blue. This is not fully established. 



THE CONSTELLATIONS. 229 

ine these and other stars and make out lists of the 
colors of all the conspicuous ones. 

The telescope shows sometimes a beautiful collection 
of stars of different colors grouped together. Such a 
cluster in the Southern hemisphere, Sir John Herschel 
said, produced on his mind the effect of a superb piece 
of fancy jewelry. Frequently a red and a blue star are 
associated together in close contrast. 

213. Twinkling of the Stars. — One of the most notice- 
able features about the stars is their twinkling. This 
is due to the different temperatures and densities of the 
strata of the atmosphere through which their light 
passes. We know this from the fact that stars near the 
horizon, the light from which traverses a greater stretch 
of atmosphere, twinkle more than those in the zenith* 
But there is some other cause connected with it, for 
some stars at the same altitude twinkle more than 
others. If Castor and Pollux be watched when they 
are near the horizon, it will be noticed that Pollux 
twinkles the most. The cause of this difference is not 
known. 

On the nights when the stars twinkle the most the 
greatest number of faint stars are visible, though for 
telescopic work such nights are apt to be poor, on ac- 
count of the unsteadiness of the atmosphere. 

20 



230 ASTRONOMY. 



Description of the Constellations. 1 

214. The Star Maps, given on pages 237-248, will 
enable the student to trace out the position of the con- 
stellations and brighter stars. As the heavens are con- 
tinually changing their position relative to the horizon 
as a result of the diurnal motion, it may be difficult at 
first to ascertain what portion is represented by any 
map. The Pole Star and the Dipper may be found, 
and the rest grouped around these. Each map con- 
tains near its edges the stars of the adjoining map, so 
they can be to some extent fitted together. Maps 1, 2, 
and 3 represent the part of the heavens around the 
North Pole in different positions. Maps 4, 5, 6, and 7 
represent a strip of the sky lying just north of the 
celestial equator, while the remaining five maps show 
a similar strip south of the equator. 

215. The circles of declination and the meridians are 
given on the maps, so that any object whose right as- 
cension and declination are known can be located. 
Also, if the position of any body is desired, it can be 
found on the map and its right ascension and declina- 
tion read off by means of the little figures on the edges. 

216. We will start with the Dipper. Take one of 
the first three maps, find the Dipper in Ursa Major, 
and hold it up so as to agree with its position in the 
sky. The following description will enable you to 
trace the other constellations around the pole, using 

1 There is no advantage in studying this section except in connec- 
tion with the maps and the heavens. Kemembering the location of 
the stars without finding them in the sky will be utterly unprofitable. 



THE CONSTELLATIONS. 231 

whichever one of the three covers the part of the sky 
you are searching in. All of the sky within 40° of the 
pole will be visible above the horizon in the latitude 
of Philadelphia, and still more farther north. 

Ursa Minor embraces Polaris. In it may be found 
the " Little Dipper," with the pole-star in the end of 
the handle. 

Cassiopeia is directly opposite Polaris from the Great 
Dipper. The conspicuous part is an irregular W, in 
which its five brighter stars are arranged. The ar- 
rangement also bears some resemblance to a chair, and 
the ancients represented Cassiopeia as a queen seated 
on a throne. If a line be drawn from the faintest star 
of the seven in the Dipper to in Cassiopeia, Polaris 
will be nearly in the middle of this line. 

The two stars y and d Cassiopeia point to a cluster 
which to the naked eye seems a mere haze of light, 
but which a small telescope resolves into a beautiful 
collection of stars. This is in the constellation Per- 
seus. Continuing this line as much farther, we come 
to several stars of medium brightness, which consti- 
tute the principal part of the constellation. /? Persei, 
or Algol, is a little farther from the pole than this 
group. Its peculiarity will be explained in the next 
chapter. 1 

The constellation Auriga adjoins Perseus on the side 



1 According to ancient mythology, Cassiopeia was the queen of 
Cepheus, King of Ethiopia. Becoming vain of her beauty, she boasted 
that she was fairer than Juno, the sister of Jupiter, or than the sea- 
nymphs. To punish her presumption, it was ordained that she should 
chain her daughter, Andromeda, whom she tenderly loved, to a desert 
rock, exposed to the fury of a sea-monster. But Andromeda was 
rescued and the monster destroyed by the great warrior Perseus. 



232 ASTRONOMY. 



away from Cassiopeia. It has two stars of considerable 
brightness, the brightest being the first-magnitude star 
Capella. 

Only one-half of the other maps are available at one 
time. The remainder represent stars below the hori- 
zon. The student, after working out the circumpolar 
stars, will have little difficulty in finding the map he 
wants. The descriptions which follow will apply to 
the heavens as they appear at 8.30 o'clock on January 
1st, April 1st, July 1st, and October 1st. 

217. Southern Constellations Visible in Winter. — Cas- 
siopeia is now west of the zenith, and the Great Dipper 
is rising in the east, 

The Milky Way extends across the heavens from the 
northwest to the southeast, passing through the zenith. 
Just on its edge, in the east, is the most brilliant con- 
stellation of the northern heavens, Orion, which on old 
star maps is represented by the figure of a hunter. Four 
bright stars form an irregular four-sided figure. One 
of these, a red star of the first magnitude, in the shoul- 
der, is Betelgeuse (« Orionis). The other first-magni- 
tude star is Rigel (/? Orionis), in the foot. Three stars 
ot the third magnitude lie in a row in the belt. Below 
the belt is another row of three fainter stars, the cen- 
tral one of which is surrounded by the famous nebula 
of Orion. On a moonless night this may be seen as a 
faint haze by the naked eye. 

Taurus lies between Orion and the zenith. The 
group Hyades is easily known by its V-shape. The first- 
magnitude star at one extremity of the V is Aldebaran. 
Farther away from Orion are the Pleiades, a little clus- 
ter which cannot be mistaken. 

Chilis Major lies opposite Orion from Taurus. The 



THE CONSTELLATIONS. 233 

very bright Sirius is its brilliant ornament. Two 
second-magnitude stars are about 11° southeast of 
Sirius. 

Procyon, in Canis Minor, makes a nearly equilateral 
triangle with Sirius and Betelgeuse. 1 

About half-way between Orion and Polaris is Ca- 
pella, the brightest star of the constellation Auriga. A 
little farther from the zenith is fi Aurigse. 

Gemini lies between Capella and the east. Castor 
and Pollux, two bright stars about 4° apart, are easily 
distinguished. Between Castor and Polaris is a barren 
part of the heavens. 

The variable star Algol will be almost exactly in the 
zenith, with the rest of Perseus a little to the north. 

Eridanus is a large constellation south of Taurus, 
which does not contain any very conspicuous stars- 

CetuSj west of Eridanus, is also a large constellation. 
It contains two stars of the second magnitude, and the 
variable Mira. 

On the western side of the zenith the most conspicu- 
ous group is the " Square of Pegasus." This consists 
of four stars in the form of a large quadrilateral, so 
arranged that two pairs point nearly to Polaris. The 
brightest of these, that one nearest the zenith, is not 
in Pegasus, but is a Andromedse. The others are a, /?, 
and y Pegasi. Between a Andromedse and Algol is £ 
Andromedse. 

The above description will also answer for 10.30 
o'clock, December 1 ; 12.30, November 1; and so on. 



1 Taurus is usually represented as a bull charging at Orion, while 
the Hunter's two dogs, Canis Major and Canis Minor, follow him 

around the sky. 

2Q* 



234 ASTRONOMY. 



218. Southern Constellations Visible in Spring. — The 
Pointers will now be nearly overhead, and Cassiopeia 
low down in the northwest. Orion and Taurus will be 
sinking to the western horizon. Sirius will be low in 
the southwest, and Procyon nearer the zenith. Castor 
and Pollux will lie between Orion and the zenith, and 
Capella between Aldebaran and Polaris. The western 
heavens have therefore already been described. Just 
east of the meridian, to the south of the zenith, is the 
constellation Leo. The most conspicuous group of 
stars is the Sickle, in the head of the Lion. At the 
end of the handle is Regulus, a first-magnitude star, 
but not a very bright one. East of the Sickle about 
16° is j8 Leonis. 

Corvus is low down in the southeast. It contains 
four stars of medium brightness, forming an irregular 
four-sided figure. 

Hydra stretches from Procyon across south of the 
zenith all the way to the southeastern horizon. One 
second-magnitude star, a Hydrae, is nearly on the 
meridian. 

Coma Berenices (Berenice's Hair) is a faint cluster 
readily seen by the naked eye, following Leo up the 
sky. Between it and the horizon is the brilliant Arc- 
tu'rus in Bootes. 

Virgo, containing the brilliant Spica, is near the 
horizon, southward from Bootes. 

This also describes the heavens at 10.30 o'clock on 
March 1, at 12.30 on February 1, and so on. 

219. Southern Constellations Visible in Summer. — The 
Milky Way extends from the north to nearly the south 
point of the horizon on the east side of the zenith. 

Of the constellations previously described, Leo will 



THE CONSTELLATIONS. 235 

be near the western horizon, and Coma Berenices just 
above it. Virgo will be low in the southwest. Bootes 
will be on the meridian, with Arcturus southwest of 
the zenith. 

Corona, a semicircle of stars, of which a is of the second 
magnitude, is on the meridian, nearly in the zenith. 

Hercules is a large constellation not containing any 
very bright stars, which adjoins Corona on the east. 

Lyra, east of Hercules, is a small but interesting con- 
stellation. The brightest star is Vega (« Lyra), a star 
of the first magnitude. Two fainter stars form with it 
an equilateral triangle. The one of these nearest the 
pole is e Lyrse, a quadruple star further described on 
page 251. The other star, <T, is double. South of these 
are p and y, between which lies the ring nebula de- 
scribed on page 253. P is a variable star (see page 256). 

Ophiuchus lies south of Hercules, and contains noth- 
ing of special interest. 

South of this, again, is Scorpio. The bright star in it 
is Antar'es. It may be known by its ruddy color and 
by the tw T o stars of the third magnitude between which 
it is set. 

Cygnns, between Lyra and the east, contains the 
" Cross." It lies exactly in the Milky Way, with the 
long arm extending in its direction. At the head of 
the cross is Deneb (a Cygni). 

Aquila, joining Cygnus on the south, contains Altair, 
of the first magnitude. It lies between two third-masr- 
nitude stars, the row pointing nearly north and south. 

Delphinus contains " Job's Coffin," a little diamond 
of stars near the eastern horizon. 

The above description -is also applicable to 10.30 
o'clock on June 1 ; 12.30 on May 1; and so on. 



236 ASTRONOMY. 



220. Southern Constellations Visible in Autumn. — The 
Dipper is now near the northern horizon. The Milky 
Way stretches through the zenith from the northeast 
to the southwest. 

All the important constellations having now been 
described, a rapid review of their positions is all that 
is necessary. 

Cygnus is in the zenith, and Lyra just west of it. 
Delphinus is a little west of the meridian, to the south 
of the zenith, and Aquila joins it to the southwest. 
Hercules and Corona are near the western horizon. To 
the east the most conspicuous object is the " Square of 
Pegasus," one of the angles of which is a Andromedse. 
Pisces joins Pegasus on the east. The bright star Fo- 
malhaut, in Piscis Australis, is low down in the south- 
southeast. 

This also describes the position of the heavens at 
10.30 o'clock on September 1 ; 12.30 on August 1 ; and 
so on. 1 

The zodiacal constellations can be found on the 
maps. Their names have been given on page 41. 
The moon and all the planets will always be found in 
these constellations. The position of the planets should 
be ascertained beforehand, as, being conspicuous ob- 
jects, the observer may lose time in searching for them 
on the charts if he mistake them for stars. 

1 Any one who desires more familiarity with the heavens than can be 
gained from this general sketch should study them with the aid of a 
large star atlas. Heis's Star Atlas is convenient and reliable ; Proc- 
tor's small Star Atlas is much cheaper. A Planisphere can be set so 
as to show the positions of all the stars with reference to the horizon 
at any time of night. An Astral Lantern is a cubical box with glass 
sides, within which is a lamp. The stars, with their names and mag- 
nitudes, show by transmitted light. It can also be set for any minute, 
and the names easily read in the dark. 



On 
< 




CM 

< 




238 




239 



< 








242 




243 



00 

< 





21* 



245 



a, 
< 




246 




247 



CM 
< 



«"fev 




248 



DOUBLE STARS. 249 



CHAPTER II. 

DOUBLE STARS.— VARIABLE STARS. — CLUSTERS AND NEB- 
ULA. — STRUCTURE OF THE UNIVERSE. 

Double Stars. 

221. Many of the stars, when examined with a tele- 
scope, are seen to he made up of two or more parts, 
which are so close together that they present to the eye 
the appearance of a single star. Sometimes the com- 
ponents are of nearly equal size, and sometimes one is 
so faint as to be seen only with large telescopes. An 
instance of the former is Castor, and among the latter 
are Sirius, Rigd, and Polaris. Sir William Herschel, 
who first carefully studied these " double stars,' 5 at first 
supposed that they happened to be nearly in the same 
line of sight, though one might be much nearer to us 
than the other. On this supposition he measured their 
distance and direction from each other, hoping that the 
motion of the earth in its orbit would cause an apparent 
change in their distance apart, and that thus he could 
determine the parallax of the nearest one. After he 
had worked at this for some time he became aware 
that there was in some cases a connection between the 
two stars entirely different from what he had expected. 
Their distance and direction from each other changed 
too rapidly to be attributed to the motion of the earth 
alone. He finally concluded that the two components 



250 ASTRONOMY. 



of a double star in many cases were revolving about 
each other. This has been established beyond doubt 
by the researches of other observers. The two or more 
stars constituting such a group are, then, members of a 
common system. They revolve about their centre of 
gravity, just as do the sun and the earth, or the earth 
and the moon (Art. 113). The force of attraction exists 
among thern, and their motions are performed in obe- 
dience to it. Several thousands of double stars are 
now known, and hundreds are added to the list every 
year. A list of such as are the most easily observed 
by the possessors of small telescopes is given in Ap- 
pendix VI. 

Those double stars in which a motion of revolution 
about each other has been certainly seen are called 
binary stars. When there are three or more in the 
group, they are called triple or multiple stars. When 
they are in the same line of sight without being bina- 
ries, they are said to be optically connected. New ob- 
servations are continually placing in the list of binary 
stars those which were previously only known to be 
optically connected. It is possible, after the observa- 
tions of several years, to calculate how large an orbit 
one of a pair of binary stars has, and how long it will 
take to complete it. The period of revolution for those 
which are determined the most accurately varies from 
twenty-five to one thousand years. So important is the 
measurement of double stars considered to be that some 
observatories give almost their whole attention to it. 
In the course of a series of years enough data will have 
accumulated to enable us to find the orbits of many 
more, and thus we shall gain additional knowledge of 
the condition of those distant suns. 



DOUBLE STARS. 251 




Fig. 55 shows the orbit which one of the compo- 
nents of y Virginis makes about the other. The num- 
bers on the diagram indicate the years when the stars 
were in that relative position. It 
will be noticed that nearly a com- 
plete revolution has been observed: 
at one end of the line we have the /82 ? 
position of one of the stars in 
1718, at the other its present posi- 
tion. 

Figs. 56 to 59 show some of 
these multiple stars. - « « 

r Fig. 5o. — Orbit of / Virginis. 

When more than two stars ex- 
ist in the group, they may still be members of the same 
system : thus, of the stars of e Lyrse, shown in Fig. 67, 
the two nearest each other perform their revolution 
in about one thousand years, the other two in about 
two thousand years ; and each pair may revolve about 
the other in an orbit of immense size and in a period 
which is many thousands of years long. This star 
can be seen double by a good eye, and an opera-glass 
shows it easily. The components are not separable 
except by a telescope. 

To measure double stars it is necessary to have an 
instrument attached to the eye end of the telescope, 
called a micrometer. This will be explained in a subse- 
quent chapter. The two measures taken are the dis- 
tance apart of the components, in seconds of arc, and 
the angle which a line joining them makes with the 
meridian through the brightest one. This angle is 
called the 'position angle, and is read all the way round 
from the north by the east from 0° to 360°. If the faint 
star were exactly north of the bright one, the position 



252 



ASTRONOMY. 



angle would be 0°. If it moved eastward around the 
bright one, the position angle would increase ; when 
south, the angle would be 180°, and when west, 270°. 
It is clear that if they revolve about each other, one of 
these measures, or both, must change in course of time. 




12 Lyncis. 
Mags. 1]/ 2i 6, 634 



£ Cancri. 
(1865.) Mags. 7, 6,734 




Ononis. 
Mags. 8, 12, 734 6, 14, 7. 

Figs. 56 to 59.— Multiple Stars. 



e Lyrae. 



If we are looking at the orbit edgewise, one star will 
seem to move backward and forward over the other, 
and only the distance will vary ; if we are in any other 
position with respect to the orbit, the angle with the 
meridian will also vary. 



VARIABLE AND NEW STARS. 253 

The double stars are sometimes beautifully colored, 
and when such is the case the colors are usually com- 
plementary 1 to each other. The larger star is most 
frequently red or orange, and the smaller one blue or 
green. Though the stars are in some instances really 
colored, it is probable that the complementary tints 
are mainly the result of contrast. 

Some stars show by their motions that they are at- 
tracted by bodies which are invisible. These are dark 
worlds, planets perhaps, but often must bear a consid- 
erable ratio to the mass of the star. The bright star 
Sirius was known by its motions to have a " compan- 
ion " long before it was found. It was at last discov- 
ered by Clark, the telescope-maker of Cambridge, 
Mass., in 1862. This is not a perfectly dark world, 
for it gives about yfo oo" as muc h light as Sirius, but it 
must weigh, in order to produce the disturbance, about 
one-fourth as much. 



Variable and New Stars. 

Nearly all the stars appear to remain of the same 
brightness night after night and year after year. 
A few of them, however, are perceptibly brighter at 
some times than at others. These are called variable 
stars. We will describe some of the most conspicuous 
of them. 



1 Colors are compleynentary when their union produces white. 
When the eve notices any color, and is then quickly turned to g. 
white or nearly white object, this object appears of a color comple- 
mentary to the first. The complementary color is in this case an 
optical illusion. 



254 ASTRONOMY. 



222. Algol. — This star is marked p in the constellation 
Perseus. In the autumn Perseus is in the northeast 
during the evening, in the winter nearly overhead, and 
in the spring in the northwest. Algol may be found 
by continuing the line joining Rigel with the Pleiades 
half as far beyond the latter. Usually its magnitude 
is 2J. For two and one-half days it continues of this 
magnitude without apparent change; then for four 
hours it fades away, till it becomes of the fourth mag- 
nitude ; here it remains for about twenty minutes, and 
then through four hours more gradually recovers its 
original brilliancy. The exact period from one mini- 
mum to the next is 2 days, 20 hours, and 49 minutes. 
Some of these minima occur in the night, and some 
when Algol is below the horizon. If the student can 
observe one, he can readily count forwards so as to 
know when to look for others. 

223. Mira. — This star is o of the constellation Cetus. 
It comes to the meridian about fifty minutes earlier 
than Algol, and three and one-half degrees south of the 
equator. It is sometimes of the eleventh magnitude, so 
that it cannot then be seen by the naked eye. It grad- 
ally increases from this to the second magnitude. After 
first becoming visible it requires about forty days to 
reach its maximum, then it fades out of sight in about 
two months, and remains invisible for seven and one- 
half months, thus passing through all its changes in 
about eleven months. This time is variable, so that 
its return cannot be exactly predicted. Its maximum 
brightness is also variable : sometimes it is almost a 
first-magnitude star ; sometimes at its brightest it does 
not exceed a fourth. It is expected to attain its great- 
est brilliancy in June, 1882, May, 1883, and so on ; but, 



VARIABLE AND NEW STARS. 255 

as the period varies from ten to twelve months, there 
may be some deviation from this. 

224. -q Argus. — This star is in the Southern hemi- 
sphere, and can never be seen in latitude north of 31° 
north. Its variations of brightness are greater than 
those of any other periodic star. It goes through its 
changes in a period of seventy years : from a star in- 
visible to the naked eye it increases almost to the 
brightness of Sirius. Its increase is not uniform, but 
numerous small fluctuations may be noticed along its 
course. The curved line of Fig. 60 shows its changes. 
The horizontal lines indicate magnitudes, and the ver- 
tical lines periods of ten years. Every seventy years it 
goes down to the sixth magnitude. At intervening times 
its brightness varies as indicated by the irregular line. 




Fig. 60. 



There are about one hundred and fifty known varia- 
bles. The following table gives the stars in which the 
variations may be most easily distinguished. Those 
of the fifth magnitude and over can be seen and studied 
by the eye, for those from the fifth to the seventh mag- 
nitude an opera-glass may be employed, while those 
below the tenth magnitude can be followed only by 
the most powerful telescopes : 



256 



ASTRONOMY. 



Name. 



5 Librae 

Persei (Algol) 

8 Cephei 

7j Aquilse 

Lyrae 

X Cygni 

R Aurigae 

o Ceti./. 

rj Argus 



Variation in 
Magnitude. 



5 to 6 

2£ to 4 

3.7 to 4.8 

3i to 5 

3i to 4% 

5 to 12 

6 to 13 
2 toll 
1 to 6 



Period. 



24 days. 
2ft ' 

n 

13 
406 
465 

It months. 

70 years. 



Many stars are known to vary their brightness slightly 
without any regular period having been discovered. 
Thus, Betelgeuse (a Orionis) is sometimes more, but 
usually less, bright than Rigel (/5 Orionis). 

225. Cause of the Variation. — "We do not certainly know 
the cause of the changes of brightness of variable stars. 
There may be a dark body like a planet revolving 
around the star, which, whenever it passes in front of 
it, cuts off a portion of its light. This is the theory 
which best explains the variations of such stars as 
Algol, which remain of the same brilliancy during most 
of the time, and at regular periods become fainter for 
a short interval. Other variables, which are gradually 
changing from one extreme to the other, are probably 
undergoing a real variation of brightness on their sur- 
faces. We have already seen that the sun is, at regu- 
lar intervals of about eleven years, largely covered with 
spots. These spots may slightly diminish its bright- 
ness, so that it can be considered a variable star with a 
period of about eleven years. If we suppose the spots 
to be greatly increased in number, so as largely to dim 
the surface of the sun, and this dimness to occur at 
regular periods, the phenomena of variable stars, as we 
notice some of them, would be accounted for. 



VARIABLE AND NEW STARS. 2bl 

226. How to observe Variable Stars. — The method de- 
vised by Argelander 1 of making observations on vari- 
able stars is as follows. Begin the watch a half-hour 
or more before the star will begin to change, and select 
two stars near the variable, one a little brighter and 
the other a little fainter. Now, if the difference between 
the brighter one and the variable is so slight that we 
could not imagine a star between the two, then the 
first star is said to be one step brighter than the varia- 
ble ; and if the variable is so much brighter than the 
other that just one star could be imagined between 
them, the variable would be brighter by two steps. If 
two stars could be supposed inserted between them, 
there is a difference of three steps ; and so on. Calling 
the comparison stars a and 6, and the variable v, the 
observation is noted thus : 

1880, May 5, 10.15 p.m., a 1 v 2 b. 

This means that a is one step brighter than v, and v 
two steps brighter than b. If the star is one that 
changes rapidly, as Algol, it should be observed every 
few minutes until its changes 'are over ; if it changes 
slowly, as Mira, once a day is often enough. The next 
observation of our star may be this : 

10.35 p.m., a 2 v 1 b; 
and the next : 

11 p.m., v = b. 

If the star grow still fainter, a new and fainter com- 



1 Argelander, 1799-1875. In charge of the Observatory of Bonn, 
Prussia. 22% 



258 ASTRONOMY. 



parison star should be taken, and the observations may 
go on, thus : 

11.30 p.m., b 1 v 4 c; 

11.55 p.m., b 2 v 3 c; 

May 6, 12.20 a.m., b Sv2c; 

12.30 a.m., b 3-4 v 1-2 c; 
12.40 a.m., b3v 2c; 
1 a.m., b 2 v 3 c; 
1.30 a.m., b 1 v 4 c. 

Here we see that the star was at its minimum about 
half-past twelve, when it was three or four steps fainter 
than b and one or two steps brighter than & The 
observations should be continued till the star has come 
back to its usual magnitude. If the exact magnitudes 
of the comparison stars are found from a catalogue 
or elsewhere, the amount of change that the variable 
makes, as well as the time of minimum 1 or maximum, 
will also become known from the observations. The 
changes are frequently irregular, and these may be 
made more striking by laying them off in curves, as 
in Fig. 60, which shows the light-curve of y Argus. 
The higher parts of the curve show when the star is 
brightest; we can see that it varies from the first 
to the sixth magnitude in seventy years, but that it 
changes irregularly. 

It must be remembered that a step is the least possi- 
ble difference, and that if there is room for an interme- 
diate star the difference is two steps. A step has been 
found to be in practice about one-tenth of a magnitude. 

1 Minimum here means the least brightness, and maximum the 
greatest. 



VARIABLE AND NEW STARS. 259 

One should not trust himself to estimate a difference 
of more than four steps, but should use another closer 
comparison star. The condition of the sky, as clear, 
hazy, moonlight, etc., should be noted with the obser- 
vation. 

227. New Stars. — New or temporary stars are such as 
suddenly blaze out and shortly afterwards disappear. 
They differ from variable stars in that their increase 
of brightness is more striking and does not return at 
regular periods. The most noted of these was seen 
by Tycho Brahe 1 in 1572. He noticed a star of the 
first magnitude where he was certain it had not existed 
a half-hour before. It continued to increase till it 
exceeded any other star in the heavens and could be 
seen at mid-day ; then it gradually faded away till it 
vanished altogether. 

There are several other cases of new stars on record. 
One of these, which appeared in 1866 and increased till 
it became of the second magnitude, was examined by 
the spectroscope : it was found that the most of the 
light was due to the presence of hydrogen gas so heated 
as to cause the great brilliancy. We have seen that 
the red prominences in the sun are composed of the 
same gas. Hence we are led to infer that the cause of 
the sudden brilliancy of the star was a great outrush 

1 A Danish astronomer, 1545 to 1601. " As a practical astrono- 
mer," says Sir David Brewster, " Tycho has not been surpassed by 
any observer of ancient or modern times. The splendor and number 
of his instruments, the ingenuity which he exhibited in inventing 
new ones, and his skill and assiduity as an observer, have given a 
character to his labors and a value to his observations which will 
be appreciated to the latest posterity." He rejected the Copernican 
theory because he supposed it to be contrary to the Bible. 



260 ASTRONOMY. 



of burning hydrogen, which, partly by its own light 
and partly by heating the surface of the star, gave rise 
to the unusual brightness. Such an outbreak on the 
sun would so raise its temperature that life on the earth 
would instantly be destroyed. A single case is not 
sufficiently conclusive to prove this theory for all new 
stars ; at all events, they are not suddenly created out 
of nothing, as was formerly supposed : the stars existed 
previously, and the brilliancy was the result of some 
change on the star itself. It is probable that some 
of these are variable stars of long period, and that we 
have observed only the one maximum. 

In 1885 a star of the 6th magnitude suddenly ap- 
peared in the midst of the Great Nebula of Andromeda 
(see page 266), and slowly faded away. 

Clusters and Nebulae. 

228. Clusters. — An observer of the heavens will no- 
tice that the stars are not uniformly distributed, but 
are frequently collected into clusters. The Pleiades and 
Hyades are examples of these. The six stars of the 
former visible to an ordinary eye become transformed 
into six hundred in the telescope. Another illustration 
is Coma Berenices, which, in the evening through the 
spring, may be seen following Leo around the heavens. 
A careful observer will also notice patches of misty 
light, which a very small telescope will convert into 
stars. One of these, which may be seen on any clear 
night, is the " Beehive cluster" in Cancer. It is about 
half-way between Regulus and Castor, a little out of the 
line joining them. Another may be found in Perseus 



CLUSTERS AND NEBULA. 



261 



by producing the line joining y and<? Cassiopeia be- 
yond the latter a distance equal to twice their distance 
apart. In the telescope it is a beautiful mass of stars. 
Others will be found named in Appendix VII. 




Fig. 61. — Cluster in Aquarius. 



Telescopes show a large number of other clusters. 
Many of these do not present any regular form. Some 
of them have a circular outline, with the stars more 
closely packed near the centre. The inference to be 
drawn from this appearance is that the cluster is in 
the shape of a globe ; hence we should look through a 
greater stretch of stars near the centre than around 
the edges. But as such collections may constitute sys- 
tems, held together by central attraction, there may 



262 



ASTRONOMY. 



also be a real condensation at the centre. The tele- 
scopic appearance of some clusters may be seen in the 
drawings on pages 261 and 262. 




Fig. 62.— Cluster in Libra. 



Fig. 63.— Cluster in Hercul 



229. Nebulce.— In addition to the clusters there may 
be seen in the telescope masses of misty light, which 




Fig. 64. — Singular Clusters. 



cannot be resolved into stars. These are nebulce. 
Some of them are clusters so far away or so faint that 



CLUSTERS AND NEBULAE. 



263 




Fig. Go. — Great Nebula of Obion, 



264 ASTRONOMY. 



the telescopic power is not sufficient to resolve them. 
It was thought for a time that as every increase of 
power resolved more and more of these into stars, 
when telescopes could be made great enough they 
would all be so resolved. But the spectroscope is able 
to tell from the character of the light whether it comes 
from a solid or a gaseous source : when directed to any 
of the resolvable nebulae, it, like the telescope, gives 
evidence of their solid condition; but when some of 
the others are studied by it, it shows that they are just 
what they seem to be, — masses of luminous gas. Such 
nebulae are not, then, collections of suns so far away 
that they seem to be clouds ; they are entirely different 
from the stars ; yet at the same time \hej give out light 
of their own, and are not rendered visible by reflecting 
the light of the suns around. They may be gradually 
condensing into suns, and perhaps serve to show the 
early stages through which all the stars have passed. 
In the course of a long period of time a sun may be 
formed from each of them. 

230. Classification. — There are over eight thousand 
known nebulae in the heavens. They are divided ac- 
cording to their form into five classes, — irregular, an- 
nular or elliptical, spiral, planetary, and nebulous stars. 
The first are the most common. The " Great Nebula 
of Orion" is an example. It may be seen with the 
naked eye on a moonless night as a faint light sur- 
rounding the star 0, the middle one of the three in the 
sword. The star itself is a multiple star, the four 
brightest components of which constitute what is called 
the. trapezium 1 of Orion (see Fig. 57). From around 

1 A trapezium is a four-sided, irregular figure. 



CLUSTERS AXD XEBULjE. 



265 




Fig. 07. — Great Nebula of Andromeda, 



266 ASTRONOMY. 



these the nebula stretches out in irregular bands and 
patches, as shown in the drawing on the preceding 
page. It envelops several of the neighboring stars, 
and seems in an indefinable manner to cover the whole 
region thereabouts. 

Ring nebulae are quite rare. One in Lyra is shown 
in 1 and 2, Fig. 66. It is situated about midway be- 
tween the stars j3 and ^, and can be seen with a small 
telescope. Sir John Herschel says, " The central 
vacuity is not quite dark, but is filled in with faint 
nebulae, like a gauze stretched over a hoop." The 
other drawings of Fig. 66 show other ring nebulae. 
Quite recently a ring 40' in diameter, but very 
faint, has been discovered in the constellation Mo- 
noceros. This is interesting from its relatively large 
size. 

Elliptic nebulae are classed with these, because they 
may be the same seen edgewise. The most conspicu- 
ous of these is the " Great Nebula of Andromeda," 
which is situated not far from (2 Andromedae. It looks 
to the naked eye like a mass of diffused light, and 
has often been mistakeu for a comet. The spectro- 
scope seems to indicate that it is not gaseous, though 
the most powerful telescope fails to resolve it into 
stars. Fig. 67 shows the appearance of this nebula 
as seen in a large telescope. Recently it has been 
photographed by an exposure of about two hours, 
and the picture, as seen in Fig. 68, presents an en- 
tirely different appearance. The annular form is 
clearly seen, and the dark rifts are evidently gaps 
between the rings like the similar forms in the case 
of Saturn. 

The camera not only gives us better pictures of 



CLUSTERS AND NEBVLM. 



267 




Fig. 68.— The Nebula in Andromeda. 
(Photographed by Mr. Roberts, December 30, 1888.) 



known objects, but it also brings to light a number of 
new nebulae. The region around the Pleiades seems 
to be full of nebulae, and many of the stars of this clus- 



268 ASTRONOMY. 



ter have nebulous wisps extending from them. These 
can be detected only on the photographic plates and 
not by direct observation. Fig. 69 shows several cir- 
cular and elliptic nebulae. 



1 2 


' 


P 


G 


7 8 

1 


9 



Fig. 69.— Circular and Elliptic Nebula. 



Spiral nebulae can be seen as such only in the largest 
telescopes. Fig. 70 represents one as it appeared in 
Lord Kosse's great reflector. 

Planetary nebulae are so called because they resemble 
a planet in appearance. In the telescope a star looks 
like a point of light, brighter but no larger than when 



CLUSTERS AND NEBULA. 269 

viewed with the eye alone. A planet, however, has its 
disk magnified when viewed in the telescope, so as to 




Fio. 70.— Spieal Nebula, 



appear of appreciable size. A planetary nebula is 
uniformly bright, and often has a well-defined outline, 



23* 



270 



ASTRONOMY. 



so that it might be mistaken for one of the outer planets 
of the solar system. 

Nebulous stars are so named because they seem to 
be surrounded by an ill-defined nebulous atmosphere. 
It is noticed in a few cases of elliptic nebulae that stars 




Fig. 71.— Dumb-Bell Nebula in Vulpecula. 

occupy positions near the two foci 1 of the ellipse. In 
other cases the stars seem to be in the centre of a neb- 



1 See page 33. 



CLUSTERS AND NEBULA. 



271 



ulous mass. These arrangements are too common to 
be the results of chance, and it is probable that there 
is some physical connection between the stars and neb- 
ulae. Such stars are often variable. A very conspicu- 
ous nebula surrounds the remarkable variable, v Arsrus. 
231. Magellanic 1 Clouds. — These are two nebulous 
objects which can be seen by the naked eye in the 
Southern hemisphere. When examined with a tele- 
scope they are shown to be made up of a collection of 



I 


-> 


: 


- . G 


$ 


|f - : -■".-- 



Fig. 72.— Double Nebula. 

nebulae, clusters, and single stars, crowded together in 
great confusion, — a kind of miniature sidereal system. 
232. Variable Nebulte. — There are nebulae, like stars, 
which vary in brightness at different times. New neb- 
ulae have also been announced where none had been 
known to exist previously. We have likewise the phe- 



1 Named from Magellan, the navigator. 



272 ASTRONOMY. 



nomena of double nebulae, the parts of which may 
revolve about each other. 



Structure of the Universe. 

233. The greatest problem which astronomers have 
ever attempted to solve is the determination of the 
shape and structure of the sidereal universe taken as 
a whole. We do not know that we have reached the 
outer bounds of the solar system ; there may be planets 
outside the orbit of Neptune ; there are probably an 
immense number of planetoids and meteors, of which 
we know nothing, inside its orbit. Since our infor- 
mation is so imperfect concerning the construction of 
the system in which we are, it might be expected that 
anything regarding the bounds of the great sidereal 
universe would be out of our reach. In most portions 
of the heavens the only effect of more powerful tele- 
scopes is to bring into view more stars and nebulae, 
without seeming to pierce through the stratum to any 
vacuity beyond. The largest glasses ever constructed 
show a thousand times as many stars as we see by the 
eye; but they reveal also faint glimmerings of light 
which tell of clusters beyond their reach. If the light 
from the nearest of the stars is years on its way to us, 
the light from some of these outlying members has 
been coming to us for centuries. These facts suggest 
to us numbers which we cannot even imagine ; the dis- 
tance to the boundaries of the universe is inconceivable, 
and to tell anything of its shape or its structure may 
well seem a hopeless problem. 

234. Distribution of the Stars. — There are, however, a 
few facts which throw a little light on the question. 



STRUCTURE OF THE UNIVERSE. 273 

Sir William Herschel, in order to aid in its solution, 
undertook a system of " star-gauging. " This consisted 
in systematically going over the heavens, pointing his 
telescope to every part, and counting the number of 
stars in the field of view. By this means he found that 
they were not distributed uniformly over the sky, but 
were arranged with some regularity with reference to 
the Milky Way. He found that the nearer to the 
Milky Way his telescope pointed, the greater was the 
number of stars he could count in its field of view at 
any one time ; and that the place in the heavens most 
barren of stars was the region that surrounded the poles 
of the Milky Way, — the points just 90° from it. Having 
ascertained with great certainty this law of distribution, 
he then took it for granted that the stars were distrib- 
uted uniformly through space ; that is, that they were all 
separated from one another by equal intervals. When 
he looked into his telescope and counted only a few 
stars, the inference would be that the system came to a 
limit soon in that direction ; if the field of view was 
crowded with stars, it might be expected that in that 
direction the system extended to a great distance, star 
beyond star, each star separated from every other by a 
distance as great as that between our sun and its nearest 
neighbor. As he had found that the stars were strewn 
more closely as he approached the Milky Way, he con- 
cluded that the universe was a flattened, lens-shaped 
mass, having its greatest extent in the direction of the 
Milky Way : when, then, we look in that direction the 
ring of light we see there indicates the great stretch of 
the universe in that plane ; when we look at right angles 
to this plane our gaze comparatively soon reaches out 
beyond its limits. This theory is wholly based on the 



274 ASTRONOMY. 



assumption of the equal distribution of the stars; if it 
be true, as seems probable, that the stars are crowded 
more closely in the plane of the Milky Way than else- 
where, it may be that the universe is no more extended 
in that direction than in others. But whatever be the 
outline of the universe, considered as a whole, Her- 
schePs investigations undoubtedly show that the greater 
number of stars are clustered near the plane of the 
Milky Way, and that we are situated in that plane, or 
near to it. The Milky Way may be such a flat disk 
as Herschel describes, or it may be a ring ; in the latter 
case w^e must suppose it to be filled inside with a looser 
company of stars, of which our sun is one. 

235. Distribution of the Nebulae. — The nebulae are ar- 
ranged very differently from the stars. While many 
clusters are in and near the Milky Way, the real ir- 
resolvable nebulae are there distributed least profusely. 
The constellation which contains the most of them is 
Virgo, which is situated as far from the Milky Way as 
possible ; on all sides of this they diminish in frequency 
with considerable regularity. The figure on the oppo- 
site page, drawn by Richard A. Proctor, shows the 
Milky Way and the distribution of the nebulae. Each 
dot is a nebula. It will be seen that they increase in 
frequency as we depart from the Milky Way. 

236. The Universe. — So far as our knowledge of the 
great sidereal system extends, which is only a very 
little way, we maj 7 , then, consider it to be either a flat 
disk or ring of stars, of which the sun is one, and that 
its greatest extent is in the direction of the Milky 
Way; while on either side of this plane are groups of 
nebulae, interspersed with a small number of stars. It 
is a very great and complicated universe. The stars in 



I 




276 ASTRONOMY. 



it are moving in all conceivable directions, and, so 
far as can now be known, do not revolve about any 
common centre, as is the case with the solar system. 1 
In all probability, around these suns are moving mul- 
titudes of dark worlds, while comets are speeding in 
all directions, messengers from one solar system to 
another. All is regulated by material laws, which keep 
every member in its place, and over all and in all rules 
the Great Lawgiver. 

237. The Nebular Hypothesis. — But the question 
comes up, What has been the past history of the uni- 
verse ? Was it created just as we study it now ? This 
is not probable. There has doubtless been a gradual 
growth to its present condition. Through what stages 
the growth has been carried we do not certainly know. 
There is a theory, commonly called the nebular hypothe- 
sis, which will account for many of the facts, but which 
seems to be disproved by others. As it has received a 
wide notoriety, we will explain it briefly here. The 
theory is that every star, with its attendant system of 
worlds, was at one time in the form of a gaseous neb- 
ula. A motion of rotation was set up in this mass. 
The central attraction w^ould tend to condense it 
towards the centre ; as it contracted in volume its ve- 
locity of rotation would increase, and the tendency of 
the parts around the equator of the mass to fly out from 
the centre would also increase. Hence there would 
be thrown ofF around the outer edge of the revolving 
nebula a ring of matter, and the remainder of the neb- 
ula would go on contracting, leaving the ring separated 



1 A theory has been proposed that Alcyone, the brightest of the 
Pleiades, is the centre of the sidereal system. There is no satisfactory 
proof of this, and astronomers consider it improbable. 



STRUCTURE OF THE UNIVERSE. 



277 



from it. When the contraction went on farther, a sec- 
ond ring would be thrown oft', and the process would 
go on till the central mass became a sun. Fig. 74 is a 
fanciful picture of the appearance of the solar system 
at one stage of development, but may be compared 
with the photograph of the Andromeda nebula of 
Fig. 68. 

The rings which had been thrown off at various times 
would also condense by radiating heat to the cooler space 




Fig. 74.— Illustration of the Nebular Hypothesis. 



around ; if the condensation was about equal all around 
the ring, a number of small masses would be formed, 
and the phenomenon of our ring of planetoids, or of the 
rings of Saturn, would be presented; if, however, one 
portion were denser than the rest, it would gradually 
attract the other parts to it, till the whole ring was 
joined in a single planetary mass. This mass might in 

24 



278 ASTRONOMY. 



its turn condense and throw off rings which would form 
the satellites. 

We can trace the possible development further. The 
planets would be at a great heat, at first being gaseous, 
and then liquid ; in course of time, by the continual 
radiation of heat, a crust would be formed on their 
surfaces which would gradually be prepared for habi- 
tation; the larger bodies, the central masses, would 
cool more slowly, and thus their worlds could have the 
benefit of their light and heat; on the other hand, the 
small moons would soon become cold and barren, as 
we know our moon to be. 

The facts in support of this theory are, — 

First. In our solar system the planets all revolve 
around the sun in one direction, and nearly in the same 
plane. The satellites in general move about their pri- 
maries in the same direction, and nearly in the same 
plane, and the planets, with the probable exceptions of 
Uranus and Neptune, turn on their axes the same way. 
If they had ever been parts of a common revolving 
body, they would of necessity show this common 
direction of rotation. 

Second. Matter in the interior of the earth is known 
to be in a liquid molten state. The heat increases as 
we descend into the earth, and the effects of heat are 
shown in the igneous and metamorphic rocks. 

Third. We see in the heavens a number of nebulse 
which seem to be in the various stages of development 
in this direction, and which the spectroscope now shows 
to be gaseous. According to the theory, these are, then, 
systems in process of growth. 



PART III 

ASTRONOMICAL INSTRUMENTS. 



238. Properties of Light, — We will now briefly con- 
sider such of the properties of light as are necessary to 
the correct understanding of the principles involved in 
the construction of the telescope and the spectroscope. 

When a body is luminous it is so on account of a 
rapid vibration of its particles. These vibrations are 
conveyed by the ether. This ether fills up all the 
space between the different bodies of the universe, and 
also exists in the pores of matter ; when these waves 
enter the eye they affect the nerve and brain in such a. 
way as to give us the sensation of light. 

Waves of light, when they pass through a substance 
of uniform density and transparency, move in straight 
lines. When they strike a smooth surface which they 
cannot penetrate, they are reflected, and bound off 4 , 
making, in the opposite direction, the same angle with 
the perpendicular to the surface which they had before 
striking. As we see an object by means of the rays 
of light which pass from it to the eye, it appears to be 
in the direction from which the light comes as it enters 
the eye. Thus, in a mirror the contents of a room seem 
to lie behind the wall, because the light from them, 
turned back by the mirror, moves from that direction, 

279 



280 ASTRONOMY. 



When these waves of light pass from one medium to 
another, transparent but of different density, they do 
not turn back, but slightly change their course. This 
change of course is termed refraction. We have shown 
in Chapter IV. the effect of the refraction of the dif- 
ferent strata of the atmosphere. If the waves pass into 
a piece of glass at an angle, the same phenomena are 
noticed; the direction changes so as to agree more 
nearly with the perpendicular to the surface. 

Another phenomenon besides refraction takes place 
when light passes from one transparent medium into 
another. The waves which make up a ray of ordinary 
light are of different lengths ; some vibrate rapidly, 
and when they reach the eye alone give the sensation 
of violet light, and some vibrate more slowly, and give 
the idea of red ; while between these are all the other 
colors of the rainbow. A ray of ordinary light con- 
tains all these colors, and when it enters obliquely a 
transparent medium of different density the short blue 
rays are turned from their course more than the longer 
red ones, and we see the rainbow colors. This is called 
dispersion. 

239. Velocity of Light. — The fact that light requires 
a certain time to pass from one point to another was 
discovered by Homer 1 in 1675. He noticed that the 
times of the eclipses and transits of Jupiter's satellites 
occurred later when the earth was on the side of its 
orbit opposite to Jupiter than when it was nearer to 
him. The first one of these points is farther from Ju- 
piter than the other by a distance equal to the diameter 
of the orbit of the earth. Thus, if EE' be the earth's 

1 Ko'mer, a Dane, 1644-1710. 



ASTRONOMICAL INSTRUMENTS. 



281 




orbit, S the sun, and J the position of Jupiter, the earth 

at E is nearer to Jupiter than at W by the distance 

EE', the diameter of its orbit. He 

therefore rightly concluded that the 

reason of the lateness was the greater 

distance the light has to pass over in 

one case than in the other. This 

lateness amounts to about sixteen 

and one-half minutes. The time it 

requires light to pass over the space 

which separates the earth from the 

sun thus becomes known, and from 

this, if we know the velocity of light, 

we can determine the distance to the 

sun. Very careful investigation has 

shown that the time necessary for 

light to pass from the earth to the sun is 498 seconds. 

By multiplying 498 by the number of miles that light 

moves in one second, we obtain the distance to the sun 

in miles. 

It therefore becomes a very important problem to 
determine the velocity of light. Several methods have 
been used, which are described in treatises on natural 
philosophy. The one which has produced the best re- 
sults is that which was first suggested by Foucault, 1 and 
which has since been carried to a great degree of per- 
fection by Michelson. 2 The outlines of the method are 
as follows. Sunlight is allowed to pass through a nar- 
row slit and fall on a mirror which is rapidly revolving ; 
from this it is reflected to another mirror, which turns 



Fig. 75. 



1 Foucault (foo-ko'), a French natural philosopher, 1816-1868. 
3 Michelson, Professor at U. S. Naval Academy at Annapolis. 

24* 



282 ASTRONOMY. 



it back to the revolving mirror, and thence to the slit. 
If the light were propagated instantaneously, it would 
be reflected back exactly to the place from which it 
started ; but, as it takes some time for it to pass twice 
between the mirrors, the revolving one has slightly 
changed position, and the reflected image will fall a 
certain distance from the slit. This small displacement 
is accurately measured, and from it can be obtained 
the time that the light requires to move from one 
mirror to the other. Great care, and numerous de- 
vices too intricate to explain here, were used to make 
the result as accurate as possible. The figures which 
it is believed most nearly represent the actual velocity 
of light are 299,940 kilometres, or 186,380 miles, per 
second. 

The distance to the sun obtained from this is 186,380 
X 498 = 92,817,240 miles. 

240. Telescopes. — There are two necessary parts of 
every telescope, — a mirror, or lens, to collect the light 
and form an image of the object, and one or more lenses 
to magnify this image. When the first of these parts 
is a mirror, it constitutes a reflecting telescope ; when a 
lens, a refracting telescope. The second part is called 
the eye-piece, because the eye is applied to it. The two 
parts are usually connected by a tube, to keep out 
side-rays. 

241. Principle of Reflectors. — The essential part of a 
reflecting telescope is a concave mirror, which collects 
rays from all parts of the object and brings them to a 
focus, forming an image of the object. The eye looks 
at this image. As many more rays of a star can fall 
on a large mirror than on the eye, a faint star will look 
just as much brighter as the surface of the mirror ex-> 



ASTRONOMICAL INSTRUMENTS. 283 

ceeds the surface of the pupil of the eye, leaving out 
some light lost by the reflection. The largest mirror of 
this kind ever made is that of Lord Rosse's telescope : 
its diameter is six feet, and it can collect 250,000 times 
as much light as the unaided eye. Its speculum, as the 
concave mirror is called, was made of a combination of 
copper and tin, which was moulded and then ground 
under water till it came exactly to the proper shape. 
Metallic specula of this kind tarnish soon, and then 
have to be taken from the tube and reground, so that 
few of them are now made. Instead of this, one side 
of a large glass disk is carefully hollowed out to the 
proper shape and covered with a very thin coating of 
silver. This does not soon tarnish, and when it does 
the silver is easily removed and a new coating applied. 
Owing to the difficulty of supporting a piece of glass 
of very large size, reflectors of this kind have not been 
made over three feet in diameter. 

242. Kinds of Reflectors. — There are three kinds of 
reflecting telescopes, depending on the situation of the 
eye-piece. The first was invented by James Gregory, 1 
the second by Sir Isaac Newton, and the third by Sir 
"William Herschel ; hence their names. 

In the Gregorian the rays, after being reflected by the 
large mirror, are collected on a smaller one, situated 
in the position of mn, Fig. 77, but so placed as to re- 
flect the rays directly back to M. The eye is placed 
back of the speculum and looks through an opening 
in it. 

In the Newtonian the second mirror is placed diago- 
nally, so that the rays are reflected out at one side of the 

1 A Scotch mathematician, 1638-1675. 



284 



ASTRONOMY. 




Fig. 76.— Newtonian Reflector Equatorially Mounted. 



ASTRONOMICAL INSTRUMENTS. 285 

tube where the eye-piece is placed. The observer looks 
at right angles to the direction of the object which he 
wishes to view. Fig. 77 shows the course of the rays 
of light through a Newtonian telescope. M is the con- 
cave speculum, and mn the diagonal mirror, or " flat/ 5 
which reflects to the eye at D. 

In the Herschelian the large mirror is tilted so as to 
bring the light to a focus at one edge of the opposite 
end of the tube. The observer is situated here, and 
has his back turned towards the object he is viewing. 




Fig. 77.— Principle of the Newtonian Keflectob. 

In the first two the small mirror cuts off a portion 
of the light which would otherwise fall on the specu- 
lum ; some light is also lost by the double reflection. 
In the third the observer's head cuts off some light, 
— less, however, than is lost in the others. The large 
telescope of Lord Rosse is Newtonian, as are also 
most of those now constructed. 

243. Principle of 'Refractors. — In refracting telescopes 
the light is collected by means of a double convex lens 
of glass. The observer looks directly towards the ob- 
ject to be viewed, as in the common spy-glass. The 
large lens is called an objective, or object-glass. 

When the early telescopes were made, a difficulty was 



286 ASTRONOMY. 



experienced from the fact that the object-glass not only 
refracted the rays and brought them to a focus, but also 
dispersed them, so that the observer saw colors sur- 
rounding the object viewed. This was corrected by 
Dollond 1 in the following manner. He made a double 
convex lens of crown glass in the usual way, and com- 
bined with it a concave lens of flint glass. The flint 
glass unites again the different-colored rays separated 
by the crown glass, while from its different quality it 
does not wholly counteract the refracting tendency of 
the convex lens. The noted opticians, Alvan Clark & 
Sons, of Cambridge, Massachusetts, make their lenses 
now as shown at A, Fig. 79, combining a double con- 
vex lens of crown with a lens of flint, of such a cur- 
vature on one side as to fit into the convexity of the 
crown, and flat on the opposite side. The best tele- 
scopic work is now done by refracting telescopes with 
their objectives arranged in this way. 

244. Eye-Pieces. — The eye-piece in a microscope for 
magnifying the image formed by the speculum or ob- 
jective. One lens would answer the purpose, but, to 
secure distinctness all around the field of view, a second 
lens is added. The amount of convexity of these lenses 
determines the magnifying power of the telescope. If 
nearly flat, the image is seen almost of its real size ; if 
more convex, the rays enter the eye so as to make a 
larger angle with one another, and the image is much 
magnified. 

Fig. 79 gives the course of rays through a refracting 
telescope. It will be seen that they cross at the focus 



1 An English optician, 1706-1761. 



ASTRONOMICAL INSTRUMENTS. 287 



B : hence such a telescope always inverts objects. This 
is a matter of no consequence with the heavenly bodies, 




Fig. 78.— Portable Newtonian Reflector. 

but when terrestrial objects are to be observed it is 
necessary to add two more lenses to turn them over 



288 ASTRONOMY. 



again. This is the only difference between an astro- 
nomical telescope and a common spy-glass. 



















i 

1 






i 
B 




A 



Fig. 79.— Illustration of the Principle of Refractors. 

245. Micrometer. — Very fine spider-webs are stretched 
across the tube in the focus. These can be seen at the 
same time with the image of the body we are observing. 
By having these movable, they can be so placed as to 
agree with the images of two stars which may be in 
the field of view, and the distance between them may 
be measured on some scale conveniently arranged for 
the purpose. Such an instrument is called a micrometer J 
and is indispensable in measuring double stars and for 
other purposes. It is so arranged that it can be taken 
off the telescope and put on at pleasure. 

246. Illuminating Power. — The advantages of tele- 
scopes are twofold: they collect a great amount of 
light, and they enable us to see a magnified image of 
the object. The first advantage will depend entirely 
on the size of the mirror or lens. This may be con- 
sidered to be a huge eye, and all the light which falls 
on it is conveyed through the eye-piece to the retina. 
Hence a great advantage of a large telescope is the 
ability to see very faint objects. HerschePs great re- 
flectors, which he made with his own hands, brought 
to his view thousands of nebulae which were not pre- 
viously known to exist. The little moons of Mars 
were never recognized till Prof. Hall saw them with 
the great refractor of the Washington Observatory. 



ASTRONOMICAL INSTRUMENTS. 289 

247. Magnifying Power. — The focal length of the ob- 
ject-glass, divided by the focal length of the eye-piece, 
expresses the magnifying power of a telescope. The 
focal length of a lens is the distance from its centre to 
the place where the image is formed. In Fig. 79, AB 
represents the focal length of the lens A. We can 
therefore increase the magnifying power either by 
lengthening this distance, or by shortening the focal 
length of the eye-piece, which is done by making it 
more convex. In early times of telescope-making the 
first method was adopted, and the instruments of the 
seventeenth century w T ere wonderfully long and un- 
wieldy. Latterly it has been deemed better to make 
the telescope moderately long, and to gain power by 
shortening the eye-piece. If the focal length of the 
object-glass were forty inches, and that of the eye-piece 
one-half inch, the magnifying power would be 40 -j- J, 
or 80. Another way to find the magnifying power is 
the following. Point the telescope to the bright sky 
and focus the eye-piece, when a small circle of light is 
observable in the eye-piece. This is merely the light 
which falls on the object-glass reduced in size by tlie 
passage through the lenses. The diameter of this circle 
divided into the diameter of the object-glass will give 
the magnifying power. 

It must be remembered that the magnifying power 
of a given eye-piece w r ill vary with the object-glass with 
w T hich it is connected ; also that there is a limit to the 
power that can be used with any size of aperture. If 
too great a power is applied, the magnified image be- 
comes indistinct. It is like looking through a pin- 
hole : everything is confused. A refractor of six inches 
aperture cannot to advantage have a power of over 

25 



290 ASTRONOMY. 



COO ; one of ten inches aperture, of over 1000 ; and so 
on. And this high power can be used only when the 
atmosphere is in a very favorable condition. 

In looking over the country on a hot day there may 
be noticed a quivering of the objects in the horizon. 
This is due to the light from these objects passing 
through strata of air which are differently heated. 
This quivering is often noticed in the telescope, and 
the higher the magnifying power the more it interferes 
with distinct vision. The nights are very few when 
the atmosphere is so steady that a very high power 
can be used to advantage. Sometimes the air in and 
around the telescope-tube becomes heated so that noth- 
ing can be done till the observatory is completely cooled 
to the surrounding temperature. The temperature in 
the telescope-room should always be as nearly as pos- 
sible the same as that outside. 

248. Equatorial Telescopes. — Small telescopes which 
require to be moved from one place to another are 
mounted on a tripod or other light stand. In obser- 
vatories it is necessary to have them permanent. All 
telescopes intended for general work are mounted 
equatorially. An equatorial telescope is shown in Fig. 
80. The advantage of this mounting is that a star can 
be easily followed as it is carried by the diurnal motion 
around the earth. The mounting consists of two axes 
at right angles to each other : one of these is pointed 
directly towards the pole of the heavens, and is called 
the polar axis ; the other is attached to this at one end, 
and is called the declination axis. The telescope turns 
on the declination axis, and with it around the polar 
axis : hence it can be pointed to any part of the sky. 
As the polar axis is parallel to the axis about which 



ASTRONOMICAL INSTRUMENTS. 



291 



the diurnal motion of the stars is performed, the tele- 
scope pointed to a star and turned on this axis alone 







>>;*> 



mi 

Fig. 8u.— Equatokial Refracting Telescope. 



will follow the star from rising to setting. This turn- 
ing can be done by clock-work so regulated as to move 



292 



ASTRONOMY. 




Fig. 81. — Lick Observatory, Mount Hamilton, California. 
(Glass made by Clark, of Cambridge, Mass. ; mounted by Waruer and Swazey, Cleveland, 0.) 



ASTRONOMICAL INSTRUMENTS. 293 

the telescope just as fast as the star moves ; that is, 
at such a rate as to make a complete revolution in a 
day. The star then will keep exactly in the field of 
view, and, if the clock works accurately, on the same 
spider-line of the micrometer, so that it can be studied 
and measured at leisure. In the figure, AB is the polar 
axis, CD is the declination axis, E is the finder, 1 F is a 
lamp so arranged as to light up the interior of the 
tube so that the spider-lines can be seen at night, G 
and G r are graduated circles upon which the distance 
of a star from the meridian and its declination may be 
read, H is the clock which turns the telescope, J is the 
eye-piece and micrometer. 

249. Transit Instrument — The transit instrument is a 
telescope mounted on a single axis which rests on two 
piers or posts. It is set so as to swing exactly in the 
meridian; hence the axis must point east-and-w T est. 
In the eye-piece of the telescope is a series of parallel 
spider-lines, which are stretched across vertically, and 
one or two horizontal lines. The pivots on which the 
axis rests are ground very carefully, so as to be exactly 
circular and of equal size. 

The use of the transit instrument is to record the 
passage of stars over the meridian, and thus find the 
true sidereal time. It will be remembered that when 
the vernal equinox crosses the meridian the sidereal 
clock indicates h. m. s. ; also that a star situated 
at this point w r ould have its right ascension h. m. 

1 A finder is a little telescope with a large field of view, by which 
to find a star. As it will embrace a large circle of the heavens, the 
star can be easily found and placed in the centre of the field of view. 
It can then be seen in the large telescope. A finder is indispensable 
to any telescope except the smallest. 

25* 



294 



ASTRONOMY. 



s. If a star pass the meridian, for instance, 1 h. 
27 m. after this, its right ascension is 1 h. 27 m. If 
this star be observed at its passage over the meridian, 




«£H£i5±£=2r- 



Fiu. 82. — Portable Transit Instrument. 



and the time recorded by an accurate sidereal clock, 
this clock will indicate 1 h. 27 m. If it do not, it is 
in error, and the amount of its error becomes known. 



ASTRONOMICAL INSTRUMENTS. 295 

AVe thus have the opportunity of correcting our side- 
real clocks if we know the exact right ascensions of 
certain stars. These ris;ht ascensions are given in the 
Nautical Almanac. The observer fixes the telescope 
to the point where the star will cross, and notes the 
time of passage over each of the vertical spider-lines. 
The average of all these times will be the time when 
it crosses the meridian, if the instrument be accurately 
adjusted; if not, certain corrections must be applied. 
This gives him the clock time of passage. He then 
compares this with the right ascension of the star as 
given in the Xautical Almanac; the difference is the 
error of the clock. From the sidereal time the mean 
solar time can be calculated. A small transit instru- 
ment is shown in Fig. 82. 

The Camera. — This must now be considered an 
astronomical instrument of value. Its advantages 
over the human eye are, — 

1st. It does not get tired. By leaving the plate ex- 
posed a long time, the telescope being kept pointed at 
the object by a driving clock, the impressions are 
strengthened, so that finally objects below the ken of 
direct vision may be observed. 

2d. It takes a quicker look. An object in rapid 
motion — a cannon-ball or flash of lightning — can be 
photographed. 

3d. It gives a perfectly accurate map of the relative 
positions of objects, and saves the labor of making 
drawings. 

Perfect steadiness and uniformity of motion on the 
part of the telescope are required, especially for long 
exposures. To see that the object keeps exactly in the 
field of view, it is customary to have telescopes which 



296 ASTRONOMY. 



are arranged for photographing made with two tubes 
side by side, attached together. While one is photo- 
graphing the object, the observer directs the motion 
through the other. 

250. Sextant. — The sextant is an instrument for 
taking angles on shipboard, or in other places where a 
fixed telescope cannot be arranged. It is shown in Fig. 
83. It consists of a graduated scale, AA, usually about 
60° in length, but divided into one hundred and twenty 
parts. Another scale works on this, which is attached 
to the arm B; on this arm is a mirror, C; another 
piece of glass, D, of which one-half is silvered and the 
other half clear, is fixed to the frame of the instrument. 




Fig. 83.— Sextant. 



The telescope E points directly to this second piece ot 
glass. If now it be desired to read an angle, — for in- 
stance, to know the height of a given star above the 
horizon at sea, — the sextant is held by the handle, so 
that the observer, looking through the clear part of the 
glass D, sees the horizon. Then he moves the index 



ASTRONOMICAL INSTRUMENTS. 297 

on the pivot till the star reflected from the mirror C 
and again from the silvered part of, D agrees with the 
horizon. The angle is then read on the scale. 

Should the observer be on land, the horizon is so 
broken that no definite point can be taken. Then it is 
necessary to read the angle between the star and its 
reflection from a vessel of mercury. This gives double 
the altitude of the star. 1 

The Astronomical Clock, and the Chronometer. — This is 
simply an unusually good common clock which beats 
seconds. The dial-plate reads up to 24 instead of 12, 
and the hour-hand makes one revolution instead of 
two in twenty-four hours. It keeps sidereal rather 
than mean solar time. Clock time may be obtained 
from it by computation. 

The error of a clock is the amount it differs from the 
true time. Its rate is its gain or loss per day. A good 
clock must have a uniform rate. 

The clock is often placed so that the observer at the 
transit instrument can hear it beat seconds. With his 
eye on the star, his ear can follow the beats of the 
clock, and the time of a star's crossing a wire can be 
determined, by a skilled observer, to one-tenth of a 
second. It is sometimes more convenient to record 
the transits by electric connection on an instrument 
called a chronograph, from which the times can be read 
at leisure. 

A chronometer is a large watch made with extreme 
care, and set in rings, so that it will preserve a horizon- 
tal position on a rolling vessel. It is used to determine 
longitude, as already explained, and, after being rated 
at an observatory, is carried by the ship on its voyage. 

1 Why is this ? 



298 ASTRONOMY. 



251. Spectrum Analysis. — The phenomena connected 
with the dispersion of light have performed a very 
important part in modern astronomical research. By 
them we have been enabled to tell the construction of 
our sun, and of all the other suns which crowd our skies; 
we have been able to say what elements compose them, 
and in w T hat form those elements .exist; we have found 
out that the nebulae are different in constitution from 
the stars ; that planets shine by reflected light, and, to 
some extent, the character of their atmospheres ; that 
some stars are moving towards us or away from us, 
and, approximately, the velocity of their motion. All 
this information has been contained in the rays of light 
which have fallen on the earth since its creation, but 
only within about a quarter of a century have we been 
able to understand it. 

We will now explain briefly the principles on which 
the science of spectrum analysis is founded. 

It has been known since the time of Sir Isaac New- 
ton that light, when passed through a prism, is divided 
into its several parts. The violet rays are turned aside 
the most, and the red least. This phenomenon is seen 
whether the light comes to the prism from the sun or 
from a burning candle or from the electric light. If 
this light falls on the prism after passing through a 
narrow slit, it is spread out into a band of colors, which 
is called a spectrum. This is nothing more than a slice 
of a rainbow cut crosswise. Now, so long as the source 
is a glowing solid or liquid, or a very greatly condensed 
gas, we obtain just such a spectrum. A heated piece 
of lime will give us exactly the same spectrum as glow- 
ing carbon, such as we have in a candle-flame. But 
suppose we pass the light from a glowing gas in ordi- 



ASTRONOMICAL INSTRUMENTS. 299 

nary state through a slit and a prism. The spectrum 
now changes. Instead of a combination of colors run- 
ning into one another we have narrow bright bands or 
lines of color, which are separated by dark intervals. 
Moreover, each gas has its own peculiar set of bright 
lines. Thus, sodium has only two yellow lines, close 
together, while the spectrum of iron is composed of 
hundreds of lines of all colors. If, then, we desire to 
know the elements of which any substance is composed, 
we may apply enough heat to vaporize these elements, 
allow the light to pass through a slit and a prism, and 
see what is the position of the bright lines formed. 
These must then be compared with known spectra. 
If we get the two sodium lines, for instance, we know 
sodium to exist in the substance examined. 

There is one other case to be considered, — that in 
which light from a solid or a liquid passes through a 
gas before it reaches the prism. Here the gas absorbs 
some of the rays of the light, and it is found that it 
absorbs exactly the same rays that it gives out when it 
is itself heated to glowing. The spectrum formed is 
then a continuous spectrum, similar to that given out 
by a solid or a liquid, but crossed by a series of narrow 
dark lines, and these lines have exactly the same posi- 
tion as the bright lines which the gas forms when self- 
luminous. Let us suppose that light from a candle 
or from white-hot iron passes through sodium vapor. 
After emerging from the prism the spectrum would be 
the ordinary spectrum, except that in the yellow por- 
tion there would be two dark lines agreeing exactly in 
position and relative character with the bright lines 
previously mentioned. If, then, we have a spectrum 
which is crossed by dark lines, we know that the light 



300 ASTRONOMY. 



comes from a solid or a liquid, or a very dense gas, and 
passes through a less bright atmosphere of gas. 

252. Fundamental Principles. — The principles of spec- 
trum analysis thus deduced are, — 

1. A glowing solid, liquid, or compressed gaseous 
body gives a continuous spectrum. 

2. A glowing gas under low pressure gives a spec- 
trum of bright lines only, each element having its 
peculiar lines. 

3. Light which comes from a glowing solid or liquid, 
or compressed gas, and passes through less bright gas, 
gives a spectrum crossed by dark lines, and these dark 
lines agree exactly in position and character with the 
bright lines given out by the same gas. 1 

253. Application to the Heavenly Bodies. — These prin- 
ciples being established by experiments with terrestrial 
substances, we have only to examine the spectra ob- 
tained from the light from the heavenly bodies to tell 
what is their constitution and composition. 

The solar spectrum is continuous, but crossed by 
dark lines. Hence we infer that the photosphere of the 
sun is solid or liquid, or a gas condensed by the enor- 
mous pressure upon it, and is surrounded by an atmos- 
phere through which the rays that reach us pass. The 

1 As the gas which is the source of light "becomes more dense and 
approaches a liquid in character, the lines of its spectrum broaden 
into bands, and when condensation is complete the bands run into one 
another and so form a continuous spectrum. It will thus be seen 
that there is no distinct line between the different kinds of spectra. 

It is supposed that the change from one kind of spectrum to 
another is due to a change in the complexity of the molecules of the 
substance examined. A simple substance gives a spectrum of lines. 
When the complexity increases by cooling down, the spectrum changes 
first to a band and then to a continuous spectrum. 



ASTRONOMICAL INSTRUMENTS. 301 

dark lines which can be seen in the spectrum agree 
with the bright lines of hydrogen, sodium, magnesium, 
and other substances on the earth. Hence we infer 
that these elements exist in the chromosphere of the 
sun; that is, that great quantities of sodium, etc., are 
burning less brightly than the sun, and the sunlight 
passes through these vapors before it reaches the earth. 

The spectra of the stars are fainter than those of the 
sun, but are of the same general character. They are 
crossed by dark lines in the same way ; the substances 
are not identical, and there is a slight diversity in the 
composition of different ones ; but all that have been 
examined show many terrestrial elements, thus proving 
that all through the universe there is the same kind of 
material. In general the red stars present a different 
kind of spectrum from the yellow, and the yellow from 
the white. Those stars which are of the same color 
have the same kind of spectra. It is considered that 
these differences indicate different stages of star-life. 
Thus, Capella and our sun are of the same color, and 
have almost exactly the same character of spectra. We 
thence infer that they are in the same condition. 

The spectra of the nebulse are some of them con- 
tinuous spectra. Such nebulae are then probably solid 
bodies, collections of suns. Others show spectra of 
bright lines only. These are then, according to our 
second principle, masses of glowing gas, which has 
not yet condensed into anything like suns. The Great 
Nebula of Andromeda belongs to the former class; 
that of Orion to the latter. 

The planets, since they are visible by reflected sun- 
light, give the same spectra as the sun, with the addi- 
tion to it of some dark lines, which are made by ab- 

26 



302 



ASTRONOMY. 



sorption of the planet's atmosphere. The spectrum of 
the moon shows none of these dark lines, but is simply 
a fainter solar spectrum, because the moon probably 
has no atmosphere, and does not give out or affect 
in any way the sunlight which it reflects. All the 
comets examined show a spectrum of bright lines on 
top of a continuous one, indicating a solid nucleus and 
a large amount of self-luminous gas surrounding it. 
This gas seems to be a hydrocarbon. 




Fig. 84.— Portion of a Spectroscope. 



254. Spectroscopes. — Fig. 84 shows the theory of the 
spectroscope. The light from the source comes in 
through one of the tubes after passing through a slit. 
It then passes through one or more prisms, — six in the 
figure, — each one farther separating the rays from one 
another. A telescope then magnifies the spectrum 
formed by the prisms. When we are examining a star, 
this whole apparatus is attached to the eye-end of a 



ASTRONOMICAL INSTRUMENTS. 303 

telescope by the tube B, so that the slit shall be in the 
focus of the object-glass. The observer looks in at A, 
and sees the light of the star, which has passed around 
through all the prisms, separated into its primary colors, 
with its dark lines in their appropriate positions. The 
more prisms there are in the train, the more the light 
will be dispersed and the longer will be the spectrum, 
but, being spread over more surface, it will be fainter. 



APPENDICES. 



LIST OF LARGE TELESCOPES. 

I.— KEFRACTORS OP OVER FIFTEEN INCHES' 
APERTURE. 



Name and Place. 


'-> a. 

< 


Maker. 


Lick Observatory, California 


36 
30 
30 
28 
27 
26 
26 
25 
23.6 
23 
20 
20 
19 
18.5 
18 
. 16 
16 
15.5 
15 
15 
15 
15 


Clark, of Cambridge, Mass. 

Clark. 

Grubb, of Dublin. 

Clark. 

T. Cooke, of England. 

Clark. 

Grubb. 

Merz & Mahler, of Germany. 

Clark. 

Fitz, of New York. 

Clark. 

Brashear. 

Clark. 

Merz & Mahler. 

Grubb. 






Yale College Observatorv 


Imperial Observatory, Vienna 


Naval Observatory, Washington 


McCormick Observatory, University of Va... 
Gateshead Observatory, England 


Paris Observatorv 




Denver Observatorv 








Private Observatory, Buffalo, N. Y 






Washburne Observatory, Madison, Wis 











26* 



305 



306 



APPENDICES. 



II.— REFLECTORS OF OYER TWO FEET APERTURE. 



Name and Place. 


a 

u a 

< 


Maker. 




72 

48 

47 

36 

31.5 

31.5 

28 
28 
28 
24 
24 


Earl of Rosse. 

Grubb. 

Martin & Eichens. 

Foucault. 
u 

H. Draper. 
Draper. 

Mr. Lassell. 


Melbourne Observatory, Australia 


Paris Observatory 


Mr. Common's Observatory, England 

Marseilles Observatory, France 


Toulouse Observatory, France 


Henry Draper's Private Observatory, Has- 
tings, N.Y 


Harvard College 


Greenwich, England 


Mr. Lassell, Maidenhead, England 


Cambridge, England ..... 





II. 
ASTRONOMICAL SYMBOLS. 

The following are the symbols and abbreviations used in ordi- 
nary almanacs. 

SIGNS OF THE PLANETS, ETC. 



0or 



© The Sun. 


cf Mars. 


« The Moon. 


% Jupiter. 


Mercury. 


h Saturn. 


9 Venus. 


S Uranus. 


£> The Earth. 


W Neptune. 


THE MOON 


'S PHASES. 



New Moon (Conjunction). 
J) First Quarter. 

SIGNS OF THE ECLIPTIC. 



O Full Moon (Opposition). 
£ Last Quarter. 



Spring 
signs. 

Summer 
signs. 



1. °f> Aries. 

2. tf Taurus. 

3. n Gemini. 

4. qz> Cancer. 

5. & Leo. 

6. *n# Virgo. 



Autumn f 7 ' =*= Libra * 
i 8. tt\, Scorpio, 



signs. 



Winter 
signs. 



9. f Sagittarius. 

10. VJ Capricornus, 

11. z$ Aquarius. 

12. X Pisces. 



APPENDICES. 307 


ABBREVIATIONS. 


Q Ascending Node. 


° ^Degrees. 


15 Descending Node. 


7 Minutes of Arc. 


6 Conjunction. 


// Seconds of Arc. 


D Quadrature. 


h Hours. 


8 Opposition. 


m Minutes of Time. 


R. A. Right Ascension. 


8 Seconds of Time. 


Dec. Declination. 





THE GREEK LETTERS. 
In astronomy, used principally to designate the different stars 
in each constellation. 

Letter. Name. 

a Alpha. 

(3 Beta. 

y Gamma. 

6 Delta. 

* Epsi'lon. 

? Zeta. 

rj Eta. 

6 Theta. 

i Io'ta. 

k Kappa. 

7i Lambda. 

(t Mu. 



Letter. Name. 


V 


Nu. 


Z 


Xi. 





Omi / kron. 


IT 


Pi. 


P 


Rho. 


a 


Sigma. 


T 


Tau. 


V 


Upsi^on. 





Phi. 


X 


Chi. 


* 


Psi. 


CJ 


Ome 7 ga. 



III. 
LENGTHS OF DAYS, MONTHS, AND YEARS. 

24 h = a mean solar day ; the ordinary day. 

23 h 56 m 4.09 s = a sidereal day ; the exact time of the earth's rotation. 
29.53088 days = a mean synodical month ; the common lunar month, 

being the time from one new moon until the next, 

or from one full moon until the next. 
27.32166 days = a sidereal month ; the time of the revolution of the 

moon about the earth. 
365.24220 days = a tropical year ; the common year, being the time 

from one vernal equinox until the next. 
365.25636 days = a sidereal year ; the time of the revolution of the 

earth about the sun. 



308 



APPENDICES. 



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APPENDICES. 



309 



PERIODIC COMETS. 



Name. 



Encke. ........ 

Tempel I .... 

Barnard 

Swift 

Brorsen 

Winnecke.... 

Tempel II.... 

D 1 Arrest 

Biela 

Wolf. 

Faye 

Denning 

Tuttle 

Pons-Brooks 
Halley 



Last 

Perihelion 

Passage. 



1891 
1888 
1889 
1891 

1890 
1891 

1891 
1890 

? 
1891 

1887 
1881 
1885 

1885 
1835 



Periodic 
Time. 



3.30 years. 
5.20 
5.35 
5.50 

5.56 
5.61 

6.00 
6.39 
6.60 
6.70 
7.41 
9.00 (?) 
13.78 

71.34 
76.37 



Notes. 



See page 198. 

Discovered in 1884. 

Seen in 1869, and not known to be pe- 
riodic till 1880. 

Discovered in 1846. 

Discovered in 1819, and not seen again 
till 1858. 

Faintest of periodic comets. 

See page 200. 

Discovered in 1884. 

Discovered in 1843. 

Discovered in 1881. 

Discovered in 1790, and not seen again 

till 1858. 
Discovered in 1885. 
See page 198. 



310 



APPENDICES. 



LIST OF NOTED DOUBLE STARS. 

COMPILED FKOM "HAND-BOOK OF DOUBLE STABS' 
OF GLEDHILL, CROSSLEY, AND WILSON. 



Name. 



51 Piscium.... 
7) Cassiopeia?.. 



Polaris 

a Piscium 

y Andromeda?.. 



i Trianguli... 

i Cassiopeia? 

yCeti 

Aldebaran.... 
Rigel 



h. m. 

U 26.2 

4L.8 

1 13.7 

1 55.8 

1 56.5 

2 5.4 
2 19.2 

2 37.1 

4 29 

5 8.8 

5 29 

5 32.7 

5 34.7 

6 39.7 

6 50.6 

7 13 
7 27 
9 11.4 

10 13.4 

12 35.6 
14 39.7 

14 45.8 

15 29.1 

16 22 

17 9.1 
17 19 5 
17 59.4 

e Lyra? 18 40.4 

19 25.8 



Orionis.. 

o- Orionis.. 
£ Orionis.. 

Sinus 



ix Canis Major is.. 
8 Geminorum — 

Castor 

38 Lyncis 

y Leonis 



y Virginis 

e Bobtis 

£ Bootis 

8 Serpentis... 

Antares 

a Herculis.... 
p Herculis. .. 
70 Ophiuchi.. 



£ Cygni 

e Draconis.. 
y Delphini., 

61 Cygni 

H Cygni 

£ Aquarii... 



R. A. 



19 48.6 

20 41.8 

21 1.4 

21 38.9 

22 22.6 



Dec. 



6 17 
57 11 

88 40 

2 11 

4L 46 

29 44 
66 51 

2 44 

16 16 
—8 201 

—5 30 

—2 40 

—2 

—16 32 

—13 53 
22 12 
32 1 

37 19 

20 27 

—0 47 

27 35 

19 36 

10 56 

—26 10 

14 32 

37 15 
2 33 

39 33 

27 42 
69 58 

15 42 

38 7 

28 12 
—0 38 



Position 
Angle. 



Dis- 
tance. 



Magnitude. 



Remarks. 



82 
145 



213 
325 



76 

(265 
1108 

291 
35 

199 



28 
5.7 

18.6 

3 

10.1 



i} 



154 

50 

342 
204 
234 
240 
112 

160 

329 

283 

186 

273 

115 

312 

78 

f 16 

[140 

56 

360 

272 

117 

118 

333 



3.9 
2.1 

7, 
3 
114 
9.2 

See 
p. 242. 



2.4 
10.1 

3 

7 

5.5 
2.9 
3.3 

5 

2.9 

4.5 

3.8 

3 

4.7 

3.7 

3.2 

3.0) 

2.5 j 
34.3 

3 
11.1 
20 

3.7 

3.4 



7.6 

9 
4 
5 



5 
4.2 



.4 

.1 8.1 



7.: 

3.5 7 
1 11.2 
1 8 

} { 



2 5.7 

1 10 

4.7 8 

3.2 8.2 

3 3.5 

4 67 

2 3.5 

3 3 

3 6.3 

4.7 6.6 

3 4 

1.5 7.7 

3 6.1 

4 5.1 
4 6 



3 5.3 

4 7.6 
4 5 
5.3 5.9 
4 5 

4 4.1 



Bi- 



White, ash. 
Yellow, purple. 

Binary. 
Yellow, white. 
White, blue. 
Yellow, blue. 

nary. 
Yellow, blue. 

Triple. Binary. 

Yellow, blue. 
Red, blue. 



Sextuple. In neb- 
ula of Orion. 

Quadruple. 

Yellow, red. Bi- 
nary. 

Small star, seen 
with difficulty. 

Yellow, blue. 

Binary. White. 
White, blue. 
Binary. Yellow, 

red. 
Yellow. Binary. 
Red, lilac. 
Pale-red, deep-red. 
White, ash. 
Red, blue. 

White. 
Binary. 

See page 241. 

Yellow, blue. 

Golden-green. 

Binary. 

WTiite. 



1 Minus sign means South Declination. 






APPENDICES. 



311 



VII. 

LIST OF NOTED CLUSTERS AND NEBULA. 

I.— CLUSTERS SEEN BY THE NAKED EYE. 

Pleiades in Taurus. 

Hyades in Taurus. 

Prresepe in Cancer. 

Coma Berenices. 

Clusters in Sword-Handle of Perseus. 



II.— CLUSTERS RESOLVABLE BY TELESCOPES. 



Constellation. 


R. A., 1880. 


Dec, 1880. 


Eemarks. 


Cassiopeia 

Auriga 

Gemini 

Coma 

Canes Venatici 


h. m. 
1 38 

5 44 

6 1 
13 7 
13 37 

15 12 

16 10 
16 37 
18 29 
18 45 
21 24 
21 27 


o / 

60 38 

32 32 

24 21 

18 48 

28 58 

2 32 

—22 42 

36 41 

24 

—6 24 

11 38 

—1 22 


Seen by two -in oh telescope. 

To naked eye, faint nebula. - 

One thousand small stars. 
Compressed cluster. 
Like a comet. 
Just visible to naked eye. 
Bright cluster. 
Fan-shaped. 
Bright cluster. 
Compressed, and resolved 
with difficulty. 


Libra 

Scorpio 

Hercules 

Sagittarius 

Antinous 

Pegasus 

Aquarius 



III.— NEBULAE. 



Constellation. 



R.A., 1880. Dec, 1880. 



Remarks. 



Great Nebula of Andromeda.. 
Great Nebula of Orion 



Ursa Major- 



Virgo . 



Canes Venatici 

Sagittarius I 

Scutum Sobieskii 

Lyra 

Dumb-Bell Nebula in Vul- 

pecula... 

Delphinus 

Aquarius 



h. m. 

36 

5 29 

f 9 39 

(11 8 

(12 20 

12 24 
(12 34 

13 25 

17 55 

18 14 

18 49 

19 54 

20 28 
20 58 



40 37 
5 29 



12 
55 
13 



50) 
40/ 
36) 

40 y 

57) 
49 



10 
27 
23 
-16 15 
32 53 

22 23 

6 59 

-11 50 



See page 253. 

See page 253. Whole neigh- 
borhood nebulous. 

Planetary nebula. 

Many nebulae about. 

Spiral nebula. 

Trifid nebula. 

Horseshoe nebula. 

Ring nebula. See page 253. 

See page 258. 

Planetary nebula. 



INDEX. 



Alexandrian astronom} T , 12. 

Algol, 254. 

Apparent motions of planets, 31. 

Auriga, 231. 

Aurora borealis, 117. 

Biela's comet, 200. 

Binary stars, 250. 

Calendar, Gregorian, 108. 

Julian, 107. 
Canis Major, 232. 
Cassiopeia, 231. 
Celestial equator, 41. 
Celestial measures, 23. 
Chaldean astronomy, 10. 
Chinese astronomy, 10. 
Chromosphere, 48. 
Clusters, 260. 
Colors of stars, 228. 
Comets, 189. 

b, 1881, 201. 

Biela's, 200. 

changes in head, 197. 

constitution, 196. 

Encke's, 199. 

Halley's, 198. 

orbits, 193. 
Conjunction, 31. 
Constellations, 217. 

description, 230. 

names, 219. 
Copernicus, 15. 
Corona, 47. 
Day, apparent solar, 101. 

civil and astronomical, 105. 



27 



Day, mean solar, 103. 

sidereal, 101. 
Day and night, 96. 
Declination, 42. 

Description of constellations, 230. 
Distribution of nebulae, 274. 

of stars, 272. 
Diurnal motion, 22. 
Double stars, 249. 
Earth, 79. 

motions, 85. 

orbit, 87. 

shape, 79. 

size, 81. 

weight and density, 83. 
Eclipses, 144. 

moon, 146. 

number, 150. 

sun, 147. 
Ecliptic, 39. 
Ellipse, 33. 
Encke's comet, 199. 
Equation of time, 104. 
Equatorial telescopes, 290. 
Equinox, 40. 
Eye-pieces, 286. 
First-magnitude stars, 222. 
Galaxy, 224. 
Galileo, 16. 
Gemini, 233. 

General view of the heavens, 18. 
Greatest elongation, 31. 
Greek astronomy, 11. 
Halley's comet, 198. 

313 



314 



INDEX. 



Heavens at the equator and poles, 24. 

Hercules, 235. 

Hipparchus, 13. 

History of astronomy, 9. 

Horizon, 19. 

Illuminating power of telescopes, 

288. 
Inferior planets, 65. 
Jupiter, 163. 

appearance, 164. 

satellites, 166. 
Kepler, 15. 
Kepler's Laws, 36. 
Latitude, 109. 
Leo, 234. 
Libration, 130. 
Light, properties, 279. 

velocity, 280. 
Longitude, 109. 

how found, 110. 
Lyra, 235. 

Magellanic clouds, 271. 
Magnifying power of telescopes, 289. 
Magnitudes of stars, 220. 
Mars, 153. 

satellites, 156. 
Mercury, 66. 
Meteors, 204. 
Meteor-watching, 210. 
Micrometer, 288. 
Milky Way, 224. 
Minor planets, 159. 
Mira, 254. 
Month, 106. 
Moon, 124. 

libration, 130. 

orbit, 125. 

phases, 126. 

physical condition, 132. 
Moons of Jupiter, 166. 

Mars, 156. 

Neptune, 185. 

Saturn, 177. 

Uranus, 182. 



Nebulse, 262. 

Nebular hypothesis, 276. 

Neptune, 183. 

New stars, 259. 

Newton, 16. 

Nodes of planets' orbits, 43. 

November and August meteors, 207. 

Nutation, 91 

Obliquity of ecliptic, 100. 

Occupations, 139. 

Opposition, 31. 

Orbits of planets, 32. 

Orion, 232. 

Parabola, 33. 

Parallax of stars, 225. 

sun, 44. 
Phenomena of Jupiter's satellites, 

168. 
Photosphere, 50. 
Planetoids, 159. 
Planets, inhabited, 18§»-^ 

orbits of, 32. 

statistics of, 35. 
Precession of equinoxes, 89. 
Proper motion of stars, 227. 
Ptolemy, 13. 

Radiant point of meteors, 209. 
Reflecting telescopes, 282. 
Refracting telescopes, 285. 
Refraction, 113. 

Relation between comets and me- 
teors, 2 1 4. 
Right ascension, 42. 
Saros, 151. 
Saturn, 173. 

rings, 175. 

satellites, 177. 
Seasons, 92. 
Sextant, 296. 
Signs of the ecliptic, 41. 
Solar prominences, 49. 
Solar system, general view, 29. 
Solstice, 40. 
Spectroscope, 302. 



INDEX. 



315 



Spectrum analysis, 298. 
Square of Pegasus, 233. 
Star-gauging, 273. 
Stars, 218. 

colors, 228. 

distances, 225. 

magnitudes, 220. 

motions, 227. 

names, 219. 

twinkling, 229. 
Statistics of sun and planets, 35. 
Structure of the universe, 272. 
Sun, 44. 

chromosphere, 48. 

corona, 47. 

distance and size of, 45. 

faculae, 59. 

heat of, 61. 

light of, 60. 

parallax of, 44. 

past and future, 63. 

photosphere, 50. 

prominences, 49. 
Sun-spots, 51. 

how to observe, 55. 



Sun-spots, periodicity of, 57. 
Taurus, 2.32. 
Telescopes, 282. 
Tides, 118. 

Time, how found, 105. 
Transit instrument, 293. 
Twilight, 116. 
Twinkling, 229. 
Universe, sidereal, 274. 
Uranus, 181. 
Ursa Major, 230. 
Ursa Minor, 231. 
Usefulness of astronomy, 26. 
Variable stars, 253. 

how to observe, 257. 
Venus, 70. 

transits of, 71. 
Virgo, 234. 
Vulcan, 65. 
Week, 106. 
Year, 107. 
Zenith, 19. 
Zodiac, 43. 
Zodiacal light, 213. 
Zones, 98. 



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and Narcotics on the Growing Body. By John C. Cutter, 
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COMPREHENSIVE ANATOMY, PHYSIOLOGY, AND 

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FIRST STEPS IN SCIENTIFIC KNOWLEDGE. 

BY PAUL BERT, 

Ex-Minister of Education in France, and Professor at La Faculti 

des Sciences de Paris. 

Adapted and Arranged for American Schools by 

W. H. GREENE, M.D., 

Professor of Chemistry in the Philadelphia Central High-School ', 

author of " Greene ' s Chemistry." 

Complete in one volume. 375 pages. 570 illustrations. 

The attention of educators is earnestly invited to this work, which has been 
written for the purpose of giving Elementary In- 
struction in Natural Science. Its sale in France, 
in less than three years, reached 500,000. There 
is scarcely a school, even in the smallest vil- 
lage, that does not use it. 

INTRODUCTION PRICE. 
Book 1. Animals, Plants, Stones, 

and Soils .... 30 cents. 
Book 2 Physics, Chemistry, Ani- 
mal Physiology, and Vegetable 
Physiology .... 36 cents. 
Complete in one volume . 60 cents 
In the American Edition such changes and 
additions have been made as were needed to 
adapt the work to American schools. The ad- 
ditions include all common and important 
American species of Animals and Plants. 
The type , plates, and illtcstrations are new ; 
the latter follow the original in size, number, 
and arrangement. The cuts of Animals were 
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TESTIMONIALS. 
" Paul Bert's book is very good. If properly used, it will arouse a great 
interest in science. The lower grades need just such a book. I will try it." — 
Col. Francis W. Parker, Principal of Cook County Normal School, Normal 
Park, III. 

" I have examined the ' First Steps in Scientific Knowledge' with some 
care, and I find it very interesting. Many things that are well known, and 
many more that are unknown to children, are set forth so clearly that they 
cannot fail to stimulate inquiry, and thus be far more useful in what they lead 
to than they are even in what they so well explain. I think such a book 
should be placed in every school." — A. P. Marble, City Superintendent of 
Public Schools, Worcester, Mass. 

" We most heartily welcome it as one of our most valuable school-books, 
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wider interest in what has been happily called ' the science of observation." — 
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" This work will be cordially welcomed by American teachers and students 
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subjects are well chosen, and the simplicity of the experiments and aptness of 
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the sciences in the lower grades of the public schools." — New England Jour- 
nal of Education. 

" So admirable a little book as this might well be made the subject of a dis- 
course on the teaching of natural knowledge, as it is one of the inost remark- 
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Worcester's Dictionary 

THE STANDARD 

IK SPELLING, PRONUNCIATION, AND DEFINITION. 




New Edition. The largest and most complete Quarto Dictionary of the 
English Language. 2126 pages. Contains thousands of words not to be found 
in any other Quarto Dictionary. Enlarged by the addition of a biographical 
dictionary of nearly 12,000 personages, and a gazetteer of the world, 
noting and locating over 20,000 places. Containing also over 12,500 new 
words, recently added, together with a table of 5000 words in general use, with 
synonymes. Illustrated with wood-cuts and full-page plates. With or without 
Denison's Patent Index. 

SCHOOL DICTIONARIES. 

Worcester's Primary Dictionary. 

Profusely Illustrated. 384 pages. i6mo. Half roan. 

Worcester's New School Dictionary. 

With numerous Illustrations. 390 pages. Half roan. 

Worcester's Comprehensive Dictionary. 

Profusely Illustrated. 688 pages. i2mo. Half roan. 

Worcester's New Academic Dictionary. 

688 pages. i2mo. Half roan. 

WORCESTER'S are the latest school dictionaries Published ; they give 
the correct usage in pronunciation ; they contain a much larger number of 
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the definitions are complete, concise, and accurate. 



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OF THE WORLD. 



A COMPLETE PRONOUNCING GAZETTEER OR GEOGRAPHICAL 
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One volume. In. perial Octavo. Embracing 2680 pages. 
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New edition. Thoroughly revised, entirely reconstructed, and 
greatly enlarged. Containing notices of over 125,000 places, and 
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Returns. It is a large octavo volume of 2680 pages, and contains 
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*' The costly and painstaking reconstruction of the work gives to the public 
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LIPPINCOTT'S 

PRONOUNCING 
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OF BIOGRAPHY AND MYTHOLOGY, 

CONTAINING MEMOIRS OF THE EMINENT PERSONS OF ALL AGES AND COUNTRIES,, 

AND ACCOUNTS OF THE VARIOUS SUBJECTS OF THE NORSE, HINDOO, AND 

CLASSIC MYTHOLOGIES, WITH THE PRONUNCIATION OF THEIR NAMES 

IN THE DIFFERENT LANGUAGES IN WHICH THEY OCCUR. 



By Joseph Thomas, M.D., LL.D., Author of Thomas's 
" Pronouncing Medical Dictionary," etc. New edition, 
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ard library." — J. H. Vincent, Chancellor Chautauqua University. 

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of whose lives you cannot learn from this work in a few words. Mechanically 
the book is strongly and finely made, and is well adapted to constant and hard 
usage, as becomes a book of reference. The directions for pronunciation are 
especially valuable. We should add the work without hesitation to the list of 
indispensables for every private library ; all public libraries will have it, of 
course." — The Literary World (Boston). 

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Chauvenet's 
Series of Mathematics. 

BY 

WILLIAM CHAUVENET, 

Late Professor of Mathematics and Astronomy in Washington 
University, St. Louis. 



CHAUVENET'S GEOMETRY. 

A Treatise on Elementary Geometry, with Appendices containing a Copious 

Collection of Exercises for the Student and an Introduction to Modern 

Geometry. 

Crown 8vo. Cloth. $1.40. 



CHAUVENET'S PLANE AND SPHERICAL 
TRIGONOMETRY. 

New and Revised Edition. 8vo. Cloth. $1.28. 



CHAUVENETS METHOD OF LEAST SQUARES. 

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servations. From the author's Manual of Spherical and Practical As- 
tronomy. 

8vo. Cloth. $1.60. 



CHAUVENET'S SPHERICAL AND PRACTICAL 
ASTRONOMY. 

Embracing the general problems of Spherical Astronomy, its special applica- 
tions to Nautical Astronomy, and the TJheory and Use of Fixed and Port- 
able Astronomical Instruments. With an Appendix on the Method of 
Least Squares. Amply Illustrated with Engravings on Wood and Steel. 

Two vols. Medium 8vo. Cloth. $7.00. 



Chauvenet's Series of Mathematics need no commendation further than 
a brief mention of their success. They have been the standard in the leading 
colleges of the country since their publication. Chauvenet's Geometry is used 
at Harvard, Yale, West Point, and Annapolis. It has been copied by nearly 
every author who has written a geometry since its appearance. 

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Reader's Reference Library. 



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" A most valuable addition to the library of the student, and to the clergy 
it ought to be specially useful." — New York Herald. 



EACH VOLUME SOLD SEPARATELY, AS FOLLOWS : 

BREWER'S HISTORIC NOTE-BOOK. 

A Dictionary of Historic Terms and Phrases. $3.50. 

THE WRITER'S HAND-BOOK. 

A General Guide to the Art of Composition and Style. $2.50. 

BREWER'S READER'S HAND-BOOK 

Of Facts, Characters, Plots, and References. $3.50. 

BREWER'S DICTIONARY OF PHRASE AND FABLE. 

Giving the Derivation, Source, and Origin of about 20,000 Common 
Phrases, Illusions, and Words that have a Tale to Tell. New edition 
{Seventeenth). Revised and corrected. $2.50. 

BREWER'S DICTIONARY OF MIRACLES. 

Imitative, Realistic, and Dogmatic. With Illustrations. $2.50. 

EDWARDS'S WORDS, FACTS, AND PHRASES. 

A Dictionary of Curious, Quaint, and Out-of-the-Way Matters #2.50. 

WORCESTER'S COMPREHENSIVE DICTIONARY. 

Revised, enlarged, and profusely illustrated. $2.50. 

ROGET'S THESAURUS. 

A Treasury of English Words. Classified and arranged so as to facili- 
tate the expression of ideas and assist in literary composition. $2.50. 

ANCIENT AND MODERN FAMILIAR QUOTATIONS. 

From the Greek, Latin, and Modern Languages. $2.50. 

SOULE'S ENGLISH SYNONYMES. 

A Dictionary of Synonymes and Synonymous or Parallel Expres- 
sions. $2.50. 



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\ 



